CONDUCTION   OF  ELECTRICITY 
THROUGH  GASES 

AND 

RADIO-ACTIVITY 
MCCLUNG 


BLAKISTON'S  SCIENCE  SERIES 

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CONDUCTION  OF  ELECTRICITY  THROUGH 
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BLAKISTON'S  SCIENCE  SERIES 

CONDUCTION  OF  ELECTRICITY 
THROUGH   GASES 

AND 

RADIO-ACTIVITY* 


A  TEXT-BOOK  WITH   EXPERIMENTS 


BY 


R.  K.  McCLUNG,  M.A.,  D.Sc. 

LECTURER     IN    PHYSICS,    UNIVERSITY    OF    MANITOBA,    WINNIPEG,    MAN. 
LATE   OF    MOUNT    ALLISON    UNIVERSITY    AND    MCGILL    UNIVERSITY 


7S  I  Uugtrattons 


PHILADELPHIA 

P.  BLAKISTON'S   SON   &   CO 

I O12    WALNUT    STREET 
1909 


8EKEBM. 


COPYRIGHT,  1909,  BY  P.  BLAKISTON'S  SON  £  Co. 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER.  PA. 


PREFACE. 


The  subject  treated  in  the  present  volume  is  one  which 
has  had  a  remarkably  rapid  growth.  Its  wonderful  develop- 
ment has  taken  place  within  a  comparatively  few  years  and 
consequently  up  to  the  present  the  study  of  it  has  been  con- 
fined almost  entirely  to  the  realm  of  research.  The  funda- 
mentals of  the  subject  however  have  now  been  pretty  thor- 
oughly investigated  and  placed  on  a  somewhat  definite  working 
basis  and  it  is  felt  that  it  has  reached  the  stage  when  it  may 
with  advantage  be  introduced,  to  some  extent,  into  the  regular 
lecture  and  laboratory  courses  of  the  colleges.  The  investiga- 
tions in  the  subject  of  gaseous  ionization  in  general  have 
increased  our  knowledge  of  physical  phenomena,  especially  in 
connection  with  electricity,  to  such  a  marked  degree  and  have 
attracted  such  wide-spread  attention  that  we  believe  every 
student  of  physics  ought  to  have  at  least  some  knowledge  of 
the  facts  in  this  connection.  Since,  as  far  as  we  know,  there 
did  not  exist  what  may  be  called  any  regular  college  text- 
book on  the  subject,  or  manual  describing  an  experimental 
course,  suitable  for  the  less  advanced  student  or  undergraduate, 
the  present  text-book  was  undertaken.  We  therefore  hope 
that  it  may  find  a  place  and  fill  a  growing  need  in  a  compara- 
tively new  field. 

Owing  to  the  existing  conditions  in  connection  with  the 
subject,  the  plan  of  a  combined  text-book  and  laboratory  man- 
ual has  been  adopted  as  one  which  seemed  to  be  most  useful 
to  the  student  in  general.  Our  aim  has  been  to  present  in 
simple  form  the  fundamental  facts  and  explanatory  theories 
accompanied,  where  possible,  by  the  description  of  suitable 
experiments  to  be  performed  in  the  laboratory  so  as  to  enable 
the  average  student  to  gain  a  working  knowledge  of  the  main 
facts  and  principles  of  the  subject  without  going  too  much  into 
details,  and  to  form  for  those  wishing  to  continue  the  work  a 
basis  for  more  advanced  study  and  original  research  if  desired. 

V 

192826 


VI  PREFACE. 

In  most  cases  the  results  to  be  expected  from  the  experi- 
ments are  given  so  that  the  student  may  know  what  to  look  for, 
as  in  this  class  of  experiments,  probably  more  than  in  almost 
any  other,  the  inexperienced  are  apt  to  be  led  astray  by  erro- 
neous results  which  arise  from  incorrect  manipulation  or  want 
of  proper  precautions.  In  addition,  this  plan  allows  the  book 
to  be  used  as  an  ordinary  text-book  without  actually  perform- 
ing the  experiments,  if  so  desired. 

To  aid  in  selecting  the  experiments  from  the  rest  of  the  text 
a  complete  list  of  them  is  given  at  the  beginning  of  the  book 
and  the  more  difficult  ones  are  indicated  therein.  The  latter 
experiments  may  with  advantage  be  omitted  from  an  ele- 
mentary first  course.  Even  in  the  case  of  the  more  elementary 
experiments  in  a  great  many  instances  they  do  not  necessarily 
depend  upon  the  preceding  ones  and  consequently  the  in- 
structor may  easily  make  a  selection,  without  breaking  the 
continuity,  to  fill  the  needs  of  a  class  requiring  a  shorter 
course. 

As  the  methods  required  in  the  experiments  in  this  class  of 
work  are  different  from  those  of  any  other  and  a  large  por- 
tion of  the  apparatus  has  to  be  specially  made,  the  descrip- 
tions of  methods  and  apparatus  have  been  given  in  some  detail 
and  a  lengthy  chapter,  namely  Chapter  II,  has  been  devoted  to 
general  descriptions  along  this  line.  In  this  connection  we 
have  also  had  in  mind  research  students  beginning  this  type 
of  work  as  it  is  found  that  they  usually  encounter  difficulties 
of  this  particular  nature  at  the  outset. 

On  account  of  the  continual  development  of  the  subject 
numerical  values  of  constants,  etc.,  frequently  suffer  change 
owing  to  improved  methods  of  determination.  In  these  cases 
an  endeavor  has  been  made  to  give  such  values  as  are  generally 
accepted  at  the  present  date. 

My  thanks  are  due  to  Mr.  G.  Dunn,  of  the  Physics  Building 
in  McGill  University  for  kind  assistance  in  preparing  some 
photographs  for  illustration. 

R.    K.    McCLUNG. 

WINNIPEG,  MAN., 
November,  1909. 


TABLE  OF  CONTENTS. 


PART  I. 
CONDUCTION  OF  ELECTRICITY  THROUGH  GASES. 

CHAPTER   I. 

PAGE 

INTRODUCTORY  EXPERIMENTS  ON  ELECTRIC  DISCHARGE i 

i.  Introduction. — 2.  Brush  Discharge. — 3.  Spark  Discharge 
from  Induction  Coil. — 4.  Relation  Between  Length  of  Spark 
and  Potential. — 5.  Effect  of  Pressure  on  the  Electric  Dis- 
charge.— 6.  Effect  of  Altering  the  Position  of  the  Anode. — 
7.  Cathode  Rays. 

CHAPTER   II. 

APPARATUS  AND  GENERAL  METHODS 7 

8.  General.  —  9.  Small  Accumulators.  —  10.  Quadrant  Elec- 
trometer.— ii.  Dolazalek  Type  of  Electrometer. — 12.  Adjust- 
ment of  Lamp  and  Scale. — 13.  Adjustment  of  Electrometer. 
14.  Screening.  —  15.  Insulation.  —  16.  Electrometer  Keys.  —  17. 
Standardization  of  Electrometer. — 18.  Connection  of  Electrom- 
eter to  other  Apparatus. — 19.  Determination  of  Capacity  of 
Electrometer  and  System. — 20.  Typical  Measurement  of  Cur- 
rent by  Electrometer. — 21.  Preliminary  Experiments  with  the 
Electrometer.  —  22.  Electroscopes.  —  23.  Illumination  of  Gold 
Leaf  and  Scale.  —  24.  Adjustment  of  Reading  Microscope. — 
25.  Calibration  of  Electroscope.  —  26.  Capacity.  —  27.  Typical 
Measurement  of  Current  by  the  Electroscope. — 28.  Prelimi- 
nary Experiments  with  the  Electroscope. — 29.  Condensers. 
— 30.  Production  of  High  Vacua. — 31.  Making  of  Air-tight 
Joints. 

CHAPTER   III. 

CATHODE  RAYS   40 

32.  Some  Properties  of  Cathode  Rays. — 33.  Magnetic  Deflec- 
tion of  Cathode  Rays. — 34.  Electrostatic  Deflection  of  Cathode 
Rays. — 35.  Cathode  Rays  Carry  Negative  Charge. — 36.  Velocity 

vii 


VI 11  CONTENTS 

PAGE 

and  Ratio  of  the  Charge  to  the  Mass  of  a  Cathode  Ray  Par- 
ticle.— 37.  Comparison  of  e/m  for  the  Cathode  Ray  Particle 
with  tfcat  for  the  Electrolytic  Ion.— 38.  Lenard  Rays.— 39. 
Canal  Rays. 

CHAPTER   IV. 

RONTGEN  RAYS.      (DESCRIPTIVE) 53 

40.  Origin  of  Rontgen  Rays. — 41.  Rontgen  Ray  Focus  Tube. 
—42.  Phosphorescent  Action  of  Rontgen  Rays.— 43.  Penetrating 
Power  of  Rontgen  Rays. — 44.  Use  of  Lead  as  a  Screen  for  the 
Rays. — 45.  Photographic  Action  of  Rontgen  Rays. — 46.  Conduc- 
tivity of  Gases  Produced  by  Rontgen  Rays. — 47.  Transportation 
and  Persistence  of  Conductivity. — 48.  Removal  of  Conductivity. 

CHAPTER   V. 

RONTGEN  RAYS.     (QUANTITATIVE  MEASUREMENTS) 64 

51.  Automatic  Focus  Tube. — 52.  Setting  up  and  Manipula- 
tion of  Rontgen  Ray  Bulb. — 53.  General  Hints  on  Making  Meas- 
urements.— 54.  Production  of  Current  Through  the  Air  by 
Rontgen  Rays. — 55.  Variation  of  Current  with  Voltage. — 56. 
Variation  of  Current  with  Distance  Between  the  Plates. — 57. 
Theory  of  lonization. — 58.  Absolute  Measure  of  Current. — 59. 
Guard-ring  Method. — 60.  Dependence  of  lonization  on  Quality 
of  the  Rays. — 61.  Absorption  of  Rontgen  Rays. — 62.  Dependence 
of  lonization  on  Pressure  of  the  Gas. — 63.  Dependence  of  loni- 
zation on  Nature  of  the  Gas. — 64.  Recombination  of  Ions. — 65. 
Diffusion  of  Ions. — 66.  Mobility  of  Ions. — 67.  lonization  by 
Collision. 


CHAPTER  VI. 

OTHER  SOURCES  OF  IONIZATION 107 

68.  Ultra-violet  Light.— 69.  Method  of  Producing  Ultra- 
violet Light. — 70.  lonization  by  Ultra-violet  Light. — 71.  Photo- 
electric Fatigue.— 72.  Incandescent  Solids.— 73.  Heated  Plati- 
num.—74.  Heated  Carbon.— 75.  lonization  from  Flames. 


CHAPTER  VII. 

IONS  AS  NUCLEI  

76.  General  Phenomena.— 77.  Expansion  Apparatus.— 78.  Pro- 


CONTENTS  IX 

PAGE 

duction  of  Clouds. — 79.  Ions  as  Nuclei. — 80.  Ions  from  Other 
Sources  as  Nuclei. — 81.  Charge  Carried  by  an  Ion. 

PART  11. 
RADIO-ACTIVITY. 

CHAPTER  VIII. 

INTRODUCTORY  EXPERIMENTS  ox  RADIO-ACTIVE  SUBSTANCES 127 

82.  Discovery  of  Radio-activity. — 83.  Warning. — 84.  Photo- 
graphic Action  of  Rays  from  Uranium. — 85.  Power  of  Uranium 
Rays  to  Discharge  an  Electrified  Body. — 86.  lonization  Current 
Produced  by  Uranium. — 87.  Other  Radio-active  Substances. — 
88.  Current  Produced  by  Other  Radio-active  Substances. — 89. 
Steady  Deflection  Method  of  Measuring  lonization  Currents. 

CHAPTER   IX. 

COMPLEXITY  OF  RADIATIONS  141 

90.  Absorption  of  Rays  from  Uranium  by  Solid  Bodies. — 91. 
Rays  from  Thorium  and  Radium. — 92.  Magnetic  Deflection  of 
£  Rays.— 93.  Magnetic  Deflection  of  a  Rays.— 94.  Electrostatic 
Deflection  of  the  Rays. — 95.  Conclusions. 


CHAPTER   X. 


GENERAL   PROPERTIES   OF  RADIATIONS 153 

96.  Methods  of  Differentiation. — 97.  Comparison  of  loniza- 
tion Produced  by  a,  £  and  7  Rays. — 98.  Photographic  Action 
of  the  Rays. — 99.  Phosphorescent  Action  of  the  Rays. — 100. 
Complexity  of  a  and  /3  Rays  from  Radium. — 101.  Absorption 
of  the  Rays  by  Solids. — 102.  Effect  of  Varying  the  Thickness  of 
Layer  of  Radio-active  Material. — 103.  Absorption  of  Rays  by 
Gases. — 104.  Effect  of  Pressure  on  lonization. — 105.  Relation 
Between  Current  and  Distance  Between  the  Plates. 


CHAPTER   XI. 

SOME  SPECIAL  PROPERTIES  AND  CONSTANTS  OF  THE  RAYS 172 

106.  Electric    Charge    Carried   by   ]8   Rays. — 107.      "  Radium 
Clock." — 108.  Electric  Charge  Carried  by  a  Rays. — 109.  Veloc- 


CONTENTS 

PAGE 


ity  and  Value  of  elm  for  j3  Rays. — no.  Velocity  and  Value 
of  elm  for  a  Rays. — in.  Mass  and  Nature  of  the  ct  Particle. 
— 112.  Energy  of  the  a  Particle. — 113.  Nature  of  the  7  Rays. 


CHAPTER   XII. 

URANIUM  X,  THORIUM  X,  ACTINIUM  X 183 

114.  Discovery  of  Uranium  X  and  Thorium  X. — 115.  Chem- 
ical Separation  of  Ur.  X. — 116.  Activity  of  Uranium  and  Ur. 
X. — 117.  Change  in  Activity  of  Uranium  and  Ur.  X. — 118. 
Thorium  and  Th.  X. — 119.  Actinium  and  Act.  X. — 120.  Theory 
of  Successive  Changes. 


CHAPTER   XIII. 

EMANATIONS    190 

121.  Discovery  of  Thorium  Emanation. — 122.  Some  Proper- 
ties of  Thorium  Emanation. — 123.  Diffusion  of  Thorium  Ema- 
nation through  Solids. — 124.  Nature  of  Radiations  Emitted  by 
Thorium  Emanation. — 125.  Decay  of  Thorium  Emanation. — 
126.  Increase  of  Current  with  Time. — 127.  Radium  Emanation. 
— 128.  Rise  of  Activity  of  Radium. — 129.  Actinium  Emanation. 
— 130.  Effect  of  Conditions  on  Emanating  Power. — 131.  Con- 
densation of  the  Emanations. — 132.  Decay  of  Emanation  at 
Low  Temperature. — 133.  Phosphorescent  Action  of  the  Ema- 
nations.— 134.  Source  of  the  Emanations. 


CHAPTER   XIV. 

EXCITED  ACTIVITY    205 

135.  Active  Deposit. — 136.  Concentration  on  Negative  Elec- 
trode.—137.  Source  of  Excited  Activity.— 138.  Decay  and  Rise 
of  Excited  Activity  from  Thorium. — 139.  Explanation  of  the 
Decay  Curves  of  the  Active  Deposit  from  Thorium. — 140.  De- 
cay and  Rise  of  the  Excited  Activity  from  Radium. — 141.  Ex- 
planation of  the  Decay  Curves  of  the  Active  Deposit  from 
Radium. — 142.  Active  Deposit  of  Slow  Change. — 143.  Active 
Deposit  from  Actinium. — 144.  Some  General  Properties  of  the 
Active  Deposits. — 145.  Some  Other  Transformation  Products. 
— 146.  Theory  of  Radio-active  Changes. — 147.  Summary  of 
Radio-active  Elements. 


CONTENTS  xi 

CHAPTER   XV. 

PAGE 

A  SPECIAL  METHOD  OF  ANALYSIS  BY  ABSORPTION  CURVES 223 

149.  Homogeneous  Sources  of  a  Rays. — 150.  Theory  of  Anal- 
ysis by  Absorption  Curves. — 151.  Experimental  Arrangement 
for  Analysis. 

CHAPTER   XVI. 

RADIO-ACTIVITY  OF  THE  ATMOSPHERE 229 

152.  Loss  of  Charge  in  Closed  Vessels. — 153.  Effect  of  Con- 
ditions on  Natural  Leak. — 154.  Excited  Activity  in  the  Atmos- 
phere.— 155.  State  of  the  Subject  of  Atmospheric  lonization. 

INDEX  233 


LIST  OF  EXPERIMENTS.1 


PART  I. 

PAGE. 

1.  Brush   discharge   from  needle  points I 

2.  Spark  discharge  from  induction  coil  by  direct  current 2 

3.  Spark  discharge  from  induction  coil  by  alternating  current...  2 

4.  Relation  between  spark  length  and  potential 2 

5.  Effect  of  pressure  on  appearance  of  electric  discharge  in  air..  3 

6.  Effect  of  pressure  on  appearance  of  electric  discharge  in  other 

gases    4 

7.  Effect  on  the  appearance  of  the  discharge  of  altering  the  dis- 

tance between  the  anode  and  cathode 5 

8.  Effect  on  the  appearance  of  the  discharge  of  placing  the  anode 

in  a  side  tube 5 

9.  Adjustment  of  lamp  and  scale  of  an  electrometer 15 

10.  Adjustment  of  an  electrometer 16 

11.  Standardization  of  an  electrometer 22 

12.  Determination  of  capacity  of  an  electrometer  system 24 

13.  Setting  up  and  adjustment  of  an  electroscope 30,  32 

14.  Calibration   of  an   electroscope 31,  32 

15.  Determination  of  capacity  of  an  electroscope 31 

16.  Measurement  of  voltage  by  electroscope 32 

17.  Measurement  of  natural  leak  of  electroscope 32 

18.  Phosphorescence  produced  by  cathode  rays 40 

19.  Opacity  of  solid  bodies  for  cathode  rays 40 

20.  Heating  effect  of  cathode  rays 41 

21.  Magnetic  deflection  of  cathode  rays 41 

22.  Electrostatic  deflection  of  cathode  rays 42 

23.  Proof  that  cathode  rays  carry  a  negative  charge 43 

24.  Determination  of  the  velocity  and  the  ratio  of  the  charge  to 

the  mass  of  a  cathode  ray  particle • .  45 

25.  Study  of   Lenard    rays 49 

26.  Study  of  Canal  rays : 51 

27.  Phosphorescent  action  of  Rontgen  rays 55 

28.  Relative  penetrating  power  of  different  types  of  Rontgen  rays  56 

1  The  experiments  in  italics  are  of  a  more  difficult  nature  than  the 
others  and  may  with  advantage  be  omitted  in  selecting  an  elementary 
or  first  course.  They  may  be  reserved  until  some  experience  has  been 
gained  in  the  use  of  the  special  apparatus  and  methods  by  means  of  the 
simpler  experiments. 

xiii 


XIV  LIST   OF   EXPERIMENTS. 

29.  Relative  absorption  of  Rontgen  rays  by  different  substances..  56 

30.  Relation  between  absorption  and  thickness  of  material 57 

31.  Photographic  action  of  Rontgen  rays 59 

32.  Discharge  of  electroscope  by  Rontgen  rays 60 

33.  Transportation  of  ions 61 

34.  Persistence  of  conductivity  in  gases 62 

35.  Removal    of    conductivity,    produced    by   Rontgen    rays,    from 

a  gas    62 

36.  Measurement  of  current  produced  by  Rontgen  rays  in  air 70 

37.  Relation  between  ionization  current  and  voltage 72 

38.  Relation  between  ionization  current  and  distance  between  the 

plates    73 

39.  Absolute  measurement  of  ionization  current 76 

40.  Dependence  of  ionization  on  quality  of  the  rays 78 

41.  Absorption  of  Rontgen  rays  by  solids ' 79 

42.  Use  of  standard  test  apparatus 80 

43.  Absorption  of  Rontgen  rays  by  liquids 82 

44.  Absorption  of  Rontgen  rays  by  gases 83 

45.  Dependence  of  ionization  on  pressure  of  the  gas 85 

46.  Dependence  of  ionization  on  nature  of  the  gas 87 

47.  Recombination  of  ions 91 

48.  Diffusion   of  ions 97 

49.  Mobility  of  ions 99 

50.  Production  of  ions  by  collision 104 

51.  Ionization   by   ultra-violet   light 109 

52.  Photo-electric  fatigue ; 1 1 1 

53.  Ionization   from   heated   platinum 112 

54.  Ionization   from  heated  carbon 1 14 

55.  Attraction  of  flames  by  charged  plates 115 

56.  Conductivity  of  flames 1 16 

57.  Conductivity  caused   by   flames 1 16 

58.  Production  of  clouds  by  expansion  in  ordinary  air 120 

59.  Production  of  clouds  by  expansion  in  air  ionized  by  Rontgen 

rays     121 

60.  Production   of   clouds   by   expansion   in   air   ionized   by  other 

sources   123 

PART  II. 

61.  Photographic  action  of  rays  from  uranium 128 

62.  Power  of  uranium  rays  to  discharge  an  electrified  body 129 

63.  Transportation  of  ions  produced  by  uranium  rays 129 

64.  Persistence  of  conductivity  produced  by  uranium 129 

65.  Removal  of  conductivity  produced  by  uranium 129 

66.  Measurement  of  ionization  current  produced  by  uranium  rays  130 


LIST   OF   EXPERIMENTS.  XV 

67.  Effect  of  distance  between  the  plates  on  ionization  current....  131 

68.  Effect  of  thickness  of  layer  of  material  on  ionization  current..  131 

69.  Test  of  constancy  of  radiations  from  uranium 132 

70.  Ionization  from  uranium  rays  in  different  gases 132 

71.  Measurement   of   current  produced   by   radium,   thorium   and 

actinium    135 

72.  Analysis  of  rays  from  uranium 141 

73.  Analysis  of  rays  from  radium  and  thorium 143 

74.  Magnetic  deflection  of  P  rays 145 

75.  Determination  of  sign  of  charge  carried  by  P  rays 146 

76.  Magnetic  deflection  of  a.  rays 147 

77.  Determination  of  sign  of  charge  carried  by  the  a  rays 150 

78.  Comparison  of  a,  P  and  7  rays  as  ionizers  in  air 154 

79.  Comparison  of  a,  P  and  7  rays  as  ionizers  in  other  gases 155 

80.  Comparison  of  photographic  action  of  a,  P  and  7  rays 157 

81.  Phosphorescent  action  of  a,  P  and  7  rays 158 

82.  Complexity  of  a  and  P  rays  from  radium 159 

83.  Absorption  of  a  rays  by  solids 159 

84.  Absorption  of  P  rays  by  solids 160 

85.  Absorption  of  7  rays  by  solids 161 

86.  Effect  of  thickness  of  layer  of  material  on  ionization 163 

87.  Absorption  of  a  rays  by  gases 165 

'88.  Effect  of  pressure  on  absorption  of  a  rays  by  gases 168 

89.  Effect  of  pressure  on  ionization  by  a  rays 169 

90.  Relation  between  current  and  distance  between  the  plates 171 

91.  Charge  carried  by  P  rays 172 

92.  Chemical  separation  of  uranium  X 183 

93.  Test  of  activity  of  uranium  X  and  uranium  residue 183 

94.  Change  in  activity  of  uranium  and  Ur.  X 184 

95.  Chemical   separation  of  thorium  X 185 

96.  Test  of  activity  of  thorium  and  thorium  X 185 

97.  Test  of  activity  of  actinium  and  actinium  X 186 

98.  Study  of  simple  properties  of  thorium  emanation 190 

99.  Diffusion  of  thorium  emanation  through  solids 192 

100.  Nature  of  radiations  from  thorium  emanation 193 

101.  Decay  of  thorium  emanation 194 

102.  Increase  of  current  from  thorium  with  time 195 

103.  Decay  of  radium  emanation 196 

104.  Rise  of  activity  of  radium  with  time 197 

105.  Effect  of  pressure  on  emission  of  emanation 198 

106.  Condensation    of    emanation 200 

107.  Test  of  rate  of  decay  of  radium  emanation  at  low  temperatures  202 

108.  Phosphorescent  action  of  emanations 202 

109.  Phosphorescent  action  of  emanations  at  low  temperatures 203 

no.  Study  of  active  deposit  from  radium  or  thorium 205 


XVI  LIST    OF    EXPERIMENTS. 

in.  Concentration  of  active  deposit  on  negative  electrode 206 

1 12.  Source  of  excited  radio-activity 207 

113.  Excited  radio-activity  proportional  to   the  amount  of  radium 

emanation    208 

114.  Decay  of  excited  radio-activity  from  thorium 208 

115.  Rise  of  excited  radio-activity  from  thorium 209 

116.  Decay  of  excited  radio-activity  from  thorium  for  short  exposure  209 

117.  Decay  of   a   ray  activity  of   active  deposit   from   radium    for 

different  times  of  exposure 211 

118.  Decay   of  ft  ray  activity  of   active   deposit   from   radium    for 

different  times  of  exposure 213 

119.  Decay  of  7  ray  activity  of  active  deposit   from   radium   for 

different  times  of  exposure 213 

120.  Analysis  of  radium  bromide  by  method  of  absorption  curves. .  226 

121.  Absorption  of  a  rays  from  radium  C 227 

122.  Analysis  of  radio  thorium  by  method  of  absorption  curves 228 

123.  Measurement  of  the  natural  ionization  in  air 229 

124.  Effect  of  conditions  on  the  natural  ionization  in  air 230 

125.  Study  of  excited  radio-activity  in  the  atmosphere 231 


OF   THE 

UNIVERSITY 


PART  I. 
CONDUCTION  OF   ELECTRICITY  THROUGH  GASES. 


CHAPTER  I. 

INTRODUCTORY  EXPERIMENTS  ON  ELECTRIC 
DISCHARGE. 

1.  Introduction. — Ordinary  gases  under  normal  conditions 
are  practically  non-conductors  of.  electricity.     When  however 
a  difference  of  potential  is  established  between  two  points  in  a 
gas  a  state  of  strain  exists  in  the  gas  which  increases  with 
increase  of  potential  until  the  gas,  no  longer  able  to  withstand 
the  strain,  breaks  down  and  a  discharge  of  electricity  passes 
between  the  points,  either  in  the  form  of  a  brush  discharge  or 
a  more  violent  disruptive  spark.     The  potential  necessary  to 
cause  such  a  discharge  is  comparatively  high,  several  thou- 
sand volts  being  required  to  produce  a  spark  of  one  centimeter 
length  in  air  at  atmospheric  pressure.     The  potential  required 
depends  upon  the  nature  and  the  pressure  of  the  gas  and  the 
shape   of  the   electrodes.     As  the   ordinary   spark   discharge 
forms  a  connecting  link  between  the  general  subject  of  elec- 
tricity (with  which  all  who  read  this  are  supposed  to  be  famil-  j 
iar)   and  the  more  particular  one  of  the  conduction  of  elec- 
tricity through  gases,  and  since  the  effect  of  gas  pressure  on 
this  discharge  has  played  such  an  important  part  in  the  devel- 
opment of  our  subject,  we  will  begin  with  a  few  introductory 
experiments  thereon. 

2.  Brush  Discharge. — Attach   a  pair  of  combs,   consisting 
of  a  set  of  sharp  needle  points  as  shown  in  Fig.  i,  to  the  poles 
of  either  a  Wimshurst  machine  without  any  Leyden  jars,  or 
an  induction  coil.     Excite  the  Wimshurst  machine  or  indue- 


2         INTRODUCTORY    EXPERIMENTS    ON    ELECTRIC   DISCHARGE 

tion  coil  and  observe  in  a  darkened  room  the  glow  surround- 
ing these  points  in  the  air.  Observe  the  in- 
crease of  this  glow  or  brush  discharge  with 
increase  of  voltage. 

3.  Spark  Discharge  from  Induction  Coil. 
— An   induction   coil    capable   of    giving   a 
FIG   i  maximum  spark  of  from  25  to  30  cm.  will 

be   found   suitable    for   these   experiments, 
although  a  slightly  smaller  one  will  serve  the  purpose. 

(a)  In  a  darkened  room  cause  a  spark  to  pass  between  the 
terminals,  either  pointed  or  spherical,  of  the  secondary  of  the 
induction  coil.    Adjust  the  current  in  the  primary  and,  starting 
with  a  spark  length  of  a  centimeter  or  two,  gradually  increase 
the  spark  gap  and  observe  carefully  the  nature  of  the  spark 
produced  in  air  at  the  various  stages.    Observe  the  irregularity 
of  path  as  the  length  increases. 

(b)  Closing  the  make-and-break  attachment  of  the  induc- 
tion coil  pass  an  alternating  current  through  the  primary  and 
observe   carefully   the  nature   of   the   discharge   between   the 
terminals  of  the  secondary.    Compare  this  discharge  with  that 
obtained   from  the  direct  current  and   observe  carefully  the 
difference. 

4.  Relation  Between  Length  of  Spark  and  Potential. — To 
determine  experimentally  the  relation  between  the  length  of 
a  spark  in  air  and  the  minimum  potential  necessary  to  produce 
it,  the.  following  apparatus  will  be  found  convenient:  (i)  A 
small  two-plate  Wimshurst  influence  machine  having  plates 
35  to  40  cm.  in  diameter;  (2)  a  battery  of  eight  or  ten  Leyden 
jars,  each  of  a  capacity  of  about  1500  electrostatic  units ; 
(3)  an  electrostatic  voltmeter  with  range  extending  from  200 
or  300  volts  to  about  30,000  volts;  (4)  an  adjustable  spark 
gap  with  spherical  terminals  mounted  so  that  the  distance 
between  them  may  be  adjusted  and  determined  accurately  by 
means  of  a  fine  screw  adjustment.  Connect  the  different  parts 
of  the  apparatus  together  as  indicated  in  Fig.  2.  See  that  the 
terminals  of  the  spark  gap  are  clean  and  well  polished.  After 
the  passage  of  each  spark  repolish  them  with  a  piece  of  cham- 


SPARK  LENGTH   AND   POTENTIAL  3 

ois  leather,  as  the  spark  destroys  the  polished  surface  at  the 
point  where  it  passes.  Keep  the  terminals  also  free  from  dust 
or  moisture.  Place  the  terminals  of  the  spark  gap  in  contact. 


J— EL 


- 


FIG.  2. 

This  may  be  determined  with  certainty  by  observing  the  volt- 
meter, for  it  will  read  zero  even  when  the  Wimshurst  machine 
is  working  if  the  spheres  are  in  contact,  since  there  is  no  poten- 
tial difference  between  them.  Separate  the  terminals  a  small 
fraction  of  a  millimeter  and  very  slowly  turn  the  influence 
machine  and  carefully  watch  the  movement  of  the  voltmeter 
needle  until  it  suddenly  returns  to  zero  when  a  spark  has 
passed.  Observe  this  maximum  reading  of  the  voltmeter  and 
also  the  length  of  spark  gap  as  indicated  by  the  screw  attach- 
ment. Gradually  increase  the  length  of  spark  gap  by  small 
stages  and  observe  the  potential  necessary  to  produce  a  spark 
in  each  case.  Then  plot  a  curve  having  lengths  in  centimeters 
for  abscissae  and  corresponding  voltages  for  ordinates. 

5.  Effect  of  Pressure  on  the  Electric  Discharge. — The  pres- 
sure of  the  gas  has  a  very  marked  effect  on  the  appearance 
and  nature  of  the  electric  discharge  through  it.  To  study  this 
the  discharge  must  take  place  within  an  air-tight  glass  vessel. 
A  glass  discharge  tube  suited  to  this  purpose  is  represented 
in  Fig.  3.  This  consists  of  a  straight  glass  tube  about  4  cm. 


4         INTRODUCTORY    EXPERIMENTS    ON    ELECTRIC   DISCHARGE 

in  diameter  and  from  30  to  40  cm.  in  length  into  the  ends  of 
which  platinum  electrodes  are  sealed,  and  to  these  are  attached 
metal  disks.  Aluminium  disks  are  very  suitable.  To  the  side 
tube,  a,  connect  an  air  pump  and  a  manometer  to  measure  the 

pressure  within  the  tube. 
Connect  the  electrodes  to 
the  terminals  of  a  Wims- 
hurst  machine  or  an  induc- 
FIG.  3.  tion  coil.  Gradually  lower 

the  pressure  and  carefully 

observe  the  change  in  appearance  of  the  discharge  within  the 
tube  as  the  pressure  changes.  Observe  also,  by  means  of  a 
voltmeter,  the  diminution  in  potential  necessary  to  produce  the 
discharge  with  decrease  of  pressure. 

At  first  the  spark  becomes  more  regular  and  uniform  between 
the  electrodes,  then  broadens  out  and  assumes  a  fuzzy  appear- 
ance of  a  bluish  color.  Observe  carefully  the  marked  appear- 
ance of  the  discharge  when  a  pressure  of  about  half  a  milli- 
meter is  reached.  The  negative  electrode  or  cathode  will  be 
found  to  be  covered  with  a  thin  layer  of  luminosity;  next  to 
this  will  be  a  dark  space  which  is  called  the  Crookes  dark 
space;  immediately  beyond  this  will  be  a  luminous  part  called 
the  negative  glow,  and  beyond  this  again  a  second  dark  region 
sometimes  called  the  Faraday  dark  space.  The  luminous 
region  between  this  point  and  the  positive  electrode  or  anode 
is  called  the  positive  column.  If  the  pressure  at  this  stage  be 
adjusted  slightly  over  a  very  small  range  and  also  if  the  current 
through  the  tube  be  varied  slightly  the  positive  column  will 
divide  up  into  alternate  light  and  dark  spaces  which  are  called 
striae.  The  appearance  of  these  strke  depends  on  a  variety 
of  conditions  of  pressure,  current,  size  of  tube,  nature  of 
gas,  etc. 

If  the  discharge  be  made  to  pass  for  any  length  of  time 
through  the  tube  the  gas  pressure  will  increase  slightly.  This 
will  be  especially  marked  in  the  case  of  a  new  tube  which  has 
not  been  used  much  before.  This  is  due  to  the  escape  of  the 
occluded  gas  from  the  walls  and  electrodes.  Repeat  these 


RELATION  OF  ANODE  TO  DISCHARGE  5 

experiments,  using  other  gases  in  the  discharge  tube  and  com- 
pare the  results  with  those  obtained  for  air. 

6.  Effect  of  Altering  the  Position  of  the  Anode. — Repeat 
these  experiments,  using  a  discharge  tube  similar  to  the  above 
only  of  greater  length,  about  80  to  100  cm.,  and  having  an 
adjustable  anode  so  that  the  distance  between  the  electrodes 
may  be  varied.  Such  a  tube  is  shown  in  Fig.  4.  The  anode 
consists  of  two  aluminium  disks,  fastened  together  by  a  cen- 
tral aluminium  rod  and  fitting  loosely  in  the  tube  so  that  it  may 


FIG.  4. 

slide  along  inside  the  tube.  Attached  to  the  bottom  is  a 
piece  of  soft  iron  so  that  the  anode  may  be  moved  along  the 
tube  by  means  of  a  magnet  from  the  outside.  The  anode  is 
connected  to  the  platinum  wire  at  the  end  of  the  tube  by 
means  of  a  very  light  flexible  coil  of  wire  which  simply  makes 
connection  but  is  not  strong  enough  to  move  the  anode. 

Observe  the  relative  proportions  of  the  space  between  the 
electrodes  occupied  by  the  different  sections  of  the  discharge 
for  different  distances  between  the  electrodes.  Note  that  when 
the  distance  between  the  electrodes  is  greater  than  a  few  cen- 
timeters an  increase  of  distance  does  not  cause  any  increase 
in  length  of  the  negative  glow  or  dark  space,  but  the  increase 
takes  place  entirely  in  the  positive  column. 

Using  a  discharge  tube  of  the  form  shown  in  Fig.  5  in 
which  the  anode  is  placed  in  a  side  tube  instead  of  in  a  direct 
line  with  the  cathode,  observe  the  appearance  of  the  discharge 
at  the  pressure  of  about  half  a  millimeter.  Note  how  the 
positive  column  bends  into  the  side  tube  but  the  position  of 
the  negative  glow  does  not  alter.  If  the  side  tube  is  not  too 
far  from  the  cathode  and  the  pressure  be  lowered  somewhat 
the  negative  glow  may  even  extend  in  a  straight  line  beyond 
this  side  tube. 


6         INTRODUCTORY   EXPERIMENTS    ON    ELECTRIC   DISCHARGE 

7.  Cathode  Rays. — Using  the  tube  shown  in  Fig.  5  lower 
the  pressure  still  more  and  carefully  observe  the  change  of 
appearance  until,  in  the  neighborhood  of  one  hundredth  of  a 

millimeter,   a  new   phenomenon 
PN   0  fya^.  makes  its  appearance.    The  posi- 
tive  column  will  begin  to  disap- 
to  PumP      pear  and  a  light  phosphorescence 
will  make  its  appearance  on  the 
FlG>  5>  walls  of  the  tube.     The  color  of 

this  phosphorescence  will  depend 

upon  the  nature  of  the  glass;  if  the  tube  is  made  of  ordinary 
soda  glass  the  phosphorescence  will  be  of  a  greenish  yellow 
color,  while  in  the  case  of  lead  glass  the  color  will  be  blue. 
This  phosphorescence  appears  to  be  produced  by  streams  of 
very  minute  particles  issuing  normally  in  straight  lines  from 
the  cathode.  They  are  consequently  called  cathode  rays  and 
have  remarkable  properties.  These  properties  will  be  dis- 
cussed in  Chapter  III. 


CHAPTER  II. 


APPARATUS    AND    GENERAL    METHODS. 

8.  The  study  of  the  conduction  of  electricity  through  gases 
and  radio-activity  during  the  last  few  years  has  developed  a 
particular   class  of   measuring   instruments   and   methods   of 
measurement  particularly  suited  to  this  class  of  work.    The  in- 
struments which  are  used  in  this  branch  of  physics  were,  for 
the  most  part,  well  known  in  principle  long  before  the  ques- 
tion  of   conductivity   in   gases   was   studied,   but   within   the 
last  few  years  they  have  reached  a  much  higher  state  of  per- 
fection and  utility.     As  some  of  these  instruments  and  the 
methods  of  measurement  used  in  this  subject  present  many 
peculiarities — sometimes    of    a    rather    troublesome    nature — 
which  one  does  not  find  in  any  other  class  of  physical  measure- 
ments in  the  laboratory  and  which  are  not  usually  given  in 
text-books  on  the  subject,  nor  even  in  the  original  papers,  and 
which  often  occasion  considerable  trouble  and  loss  of  time  to 
one  beginning  the  subject,  a   somewhat   detailed   description 
of  apparatus  and  general  methods  of  procedure  will  be  given 
in  this  chapter. 

9.  Small  Accumulators. — One  of  the  most  necessary  parts 
of  the  equipment  for  this  work  is  a 

perfectly  reliable  source  of  potential 
which  may  be  adjusted  at  will  from 
a  few  volts  to  six  or  eight  hundred 
volts.  This  range  will  serve  for  most 
purposes,  although  it  will  be  found 
convenient  in  some  cases  if  the  range 
can  be  extended  to  a  thousand  volts. 
This  steady  potential  is  best  obtained 
from  a  set  of  small  accumulators. 
Since  the  current  to  be  drawn  from 
this  steady  voltage  is  never,  except  in  very  rare  cases,  more 

7 


FIG.  5a. 


8  APPARATUS   AND   GENERAL    METHODS 

than  a  very  small  fraction  of  an  ampere,  the  storage  cells  used 
to  furnish  it  need  not  be  of  large  capacity.  They  are  identical 
in  principle  with  the  ordinary  large  accumulators  and  are 
usually  made  of  plates  of  about  18  to  20  sq.  cm.  in  area  con- 
tained in  correspondingly  small  glass  vessels.  There  are  dif- 
ferent varieties  of  these  small  cells  furnished  by  different 
makers.  One  very  convenient  type  is  shown  in  Fig.  50  on  the 
preceding  page. 

These  cells  should  be  set  up  in  wooden  trays,  lined  with 
insulating  material  such  as  mica,  in  small  lots  in  series  of  not 
more  than  ten  or  twelve,  with  their  terminals  connected  to 
metal  mercury  cups,  and  these  separate  lots  may  be  combined 
in  larger  trays  so  that  any  number  may  be  readily  connected 
temporarily  in  series  as  desired. 

The  care  of  these  cells  is  a  very  important  factor  in  gaining 
efficient  service  from  them.  They  should  be  kept  in  a  closed 
cabinet  to  preserve  them  free  from  dirt,  but  this  cabinet 
should  be  so  arranged  that  each  small  lot  can  be  separately 
disconnected  and  removed  so  as  to  be  attended  to  individually, 
as  connections  and  individual  cells  are  very  apt  to  go  wrong 
through  corroding,  breaking  and  other  causes.  Lead  wires 
covered  with  rubber  are  usually  more  satisfactory  as  perma- 
nent connectors  than  copper  or  other  metal.  The  cells  should 
be  kept  clean  and  well  insulated.  They  should  be  kept  charged 
up  to  their  full  voltage  and  never  under  any  circumstances 
should  sufficient  current  be  drawn  from  them  to  cause  them 
to  fall  more  than  12  or  15  per  cent,  below  their  normal  voltage 
before  recharging.  They  ought  to  be  charged  regularly,  and 
even  when  they  are  not  used  for  a  considerable  length  of  time 
it  is  advisable  to  discharge  them  slowly  through  a  suitable 
resistance  at  intervals  and  recharge  them  again  during  the 
time  they  are  idle.  Care  should  be  taken  to  charge  them  at  the 
proper  rate,  as  too  large  a  charging  current  is  apt  to  injure 
them.  The  charging  rate  is  usually  furnished  by  the  makers 
with  each  particular  type  of  cell. 

Since  the  voltage  used  in  most  cases  is  quite  large,  extreme 
care  should  be  taken  to  prevent  a  short  circuit  of  the  cells,  as 


QUADRANT  ELECTROMETER  9 

such  of  course  is  injurious  to  the  cells  and  may  be  disastrous 
to  other  valuable  apparatus  involved. 

If  these  precautions  are  observed  and  the  cells  given  careful 
attention  they  should  present  no  difficulties. 

10.  Quadrant  Electrometer. — In  most  text-books,  even  of  a 
somewhat  advanced  nature,  the  quadrant  electrometer  is  usu- 
ally dismissed  with  a  more  or  less  brief  description  and  conse- 
quently is  not  as  well  known  in  detail  as  it  might  be.  It  is 
also  somewhat  neglected  in  many  laboratory  courses.  As  the 
use  of  the  instrument  is  so  extensive  and  important  in  the 
class  of  work  in  hand,  and  as  it  has  the  reputation,  not  wholly 
undeserved,  of  being  a  difficult  and  troublesome  instrument  to 
use,  a  somewhat  detailed  account  will  be  given  here. 

The  electrometer  consists  of  the  following  essential  parts 
which  are  common  to  every  type:  A  circular  metal  box,  fixed 
in  a  horizontal  position  and  having  a  small  vertical  central 
hole  through  it,  is  divided  by  very  narrow  saw  cuts  into  four 
quadrants  as  indicated  in  Fig.  6.  Each  of  these  quadrants 
rests  on  an  insulating  support.  Within  these  quadrants,  mid- 
way between  the  upper  and  lower  plates,  is  suspended  by  a 
very  light  fiber  suspension  a  flat 
dumbbell-shaped  needle  made  of 
aluminium  or  other  light  con- 
ducting substance.  The  diag- 
onally opposite  quadrants  are 
connected  together  as  shown  in 
Fig.  6.  If  the  needle  is  hung 
symmetrically  with  regard  to 
the  quadrants  as  shown  in  the 
diagram,  and  if  it  is  charged  to 
a  positive  potential  and  a  differ- 
ence of  potential  be  established  between  the  two  pairs  of 
quadrants,  the  needle  will  be  attracted  by  one  pair  and  repelled 
by  the  other  pair  and  will  rotate  in  a  horizontal  plane.  The 
amount  of  attraction  and  repulsion  will  depend  upon  the  dif- 
ference of  potential  between  the  quadrants  and  the  potential 
of  the  needle  and  consequently  the  amount  of  rotation  of  the 


10 


APPARATUS   AND    GENERAL    METHODS 


needle   will   depend   upon   these    factors   and   also   upon   the 
torsional  rigidity  of  the  suspension. 

The  remaining  parts  vary  in  different  types  of  instruments. 
In  the  older  forms  and  even  in  some  of  the  modern  types  of 
instrument  the  needle  is  connected  to  a  small  condenser  usually 
in  the  form  of  a  Leyden  jar  to  increase  the  capacity  of  the 
needle,  so  that  any  small  leakage  of  the  charge  from  the  needle 
will  not  alter  its  potential  as  much  as  if  its  capacity  were 
small.  Connection  between  the  needle  and  inside  coating  of 
the  Leyden  jar  is  made  by  allowing  the  central  rod  of  the 
needle  to  extend  below  the  quadrants  and  dip  into  sulphuric 
acid  contained  in  the  Leyden  jar.  The  acid  also  acts  as  a  drying 


W 


0 


FIG.  7. 

agent  to  keep  the  instrument  free  from  moisture.  If  the  wire 
dipping  into  the  acid  is  bent  into  a  small  loop  as  shown  in 
Fig.  7  the  acid  also  serves  to  damp  the  motion  of  the  needle 
so  that  it  may  turn  more  slowly  and  at  a  uniform  speed  when 
the  quadrants  are  receiving  a  charge  at  a  uniform  rate. 


UNIVERSITY 


ELECTROMETER  1  1 

There  are  difficulties  connected  with  this  Leyden  jar.  It  is 
very  difficult  to  secure  perfect  insulation  between  the  coatings 
for  large  charges  on  the  needle  and  its  potential  does  not  there- 
fore remain  constant,  which  is  a  serious  drawback  in  accurate 
measurements.  A  more  satisfactory  form  of  condenser,  de- 
vised by  Strutt  and  shown  in  Fig.  7,  in  which  the  dielectric 
is  ebonite  or  sulphur  —  which  are  much  better  insulators  than 
glass  —  may  be  used.  A  circular  plate  of  ebonite  about  I  cm. 
thick  and  6  or  7  cm.  in  diameter  is  recessed  until  the  central 
part  is  only  about  0.5  mm.  thick.  It  rests  on  a  metal  plate 
connected  to  earth  and  another  metal  plate  c  fits  into  the 
recess.  To  make  contact  between  this  plate  c  and  the  needle 
a  small  glass  vessel  containing  sulphuric  acid,  into  which  dips 
the  rod  of  the  needle,  rests  on  this  plate  and  a  wire  d  makes 
contact  between  the  acid  and  plate.  If  the  surface  of  the 
ebonite  deteriorates  it  may  easily  be  cleaned  by  removing  the 
surface  by  fine  emery  paper  or  by  scraping. 

The  sulphuric  acid  in  any  of  these  forms  of  condenser  often 
presents  a  troublesome  difficulty.  After  standing  for  a  con- 
siderable time  a  film  forms  on  the  surface  and  the  surface 
tension  between  this  film  and  the  rod  of  the  needle  affects  the 
motion  of  the  needle  in  a  troublesome  manner.  This  may  be 
overcome  by  stirring  the  acid  thoroughly  or  by  renewing  it. 
As  the  acid  absorbs  moisture  rapidly  care  must  be  taken  that 
it  does  not  overflow  the  vessel  and  ruin  the  instrument. 

To  secure  sensitiveness  in  the  instrument  the  needle  should 
be  made  as  light  as  possible  consistent  with  rigidity.  It  may 
be  made  of  thin  aluminium  just  thick  enough  to  maintain  its 
shape  rigidly  or  it  may  be  made  of  paper  silvered  on  the  sur- 
face to  make  it  conducting.  Many  of  even  the  older  types 
of  instrument  may  be  made  quite  sensitive  by  fitting  them  with 
a  light  needle.  The  needle  should  be  perfectly  symmetrical 
in  shape. 

The  suspension  for  the  needle  should  be  very  light  if  great 
sensitiveness  is  required.  The  most  satisfactory  non-conducting 
suspensions  are  fine  quartz  fibres.  These  can  either  be  made 
in  the  laboratory  or  may  be  obtained  from  instrument  makers 


12  APPARATUS  AND  GENERAL   METHODS 

in  different  grades  of  thickness.  The  fibre  may  be  attached 
at  the  ends  to  the  metal  parts  of  the  instrument  by  a  minute 
portion  of  hard  wax.  Care  must  be  observed  that  the  wax 
used  is  of  a  comparatively  rigid  nature  so  as  to  prevent  any 
movement  at  the  joint  when  the  suspension  is  under  torsion. 

To  communicate  the  charge  to  the  insulated  needle  it  is 
convenient  to  use  a  wire  W  (Fig.  7),  passing  through  an 
ebonite  plug  in  the  top  of  the  instrument  and  bent  twice  at 
right  angles,  so  that  the  lower  arm  may  be  turned  round  in 
contact  with  the  stem  of  the  needle.  The  upper  arm  can  then 
be  placed  in  contact  with  a  set  of  small  accumulators  or  other 
source  of  potential. 

To  maintain  the  potential  of  the  needle  constant  we  would 
strongly  recommend  using  a  conducting  suspension  instead  of 
the  insulating  quartz  by  means  of  which  the  needle  can  be  kept 
permanently  connected  to  a  set  of  small  accumulators  or  other 
cells.  A  very  light  phosphor-bronze  galvanometer  suspension 
will  be  found  very  satisfactory.  The  potential  of  the  needle 
may  thus  be  kept  perfectly  constant  and  may  be  altered  by  any 
definite  amount  at  will  and  the  sensitiveness  may  be  maintained 
the  same  day  after  day.  Using  this  method  the  condenser  may 
be  discarded  and  all  the  accompanying  difficulties  of  insula- 
tion, etc.,  eliminated. 

The  mathematical  theory  of  the  electrometer  generally  found 
in  text-books  is  somewhat  faulty.  The  result  is  usually  given 
about  as  follows:  If  V^,  Vz  and  V  ^  are  the  potentials  of  the 
two  pairs  of  quadrants  and  the  needle  respectively,  and  F  is 
the  torsional  couple  for  a  deflection  of  unit  angle,  then  for  a 
deflection  6  the  relation  given  is 


where  a  is  a  constant.    It  follows  from  this  that  if  Fn  be  very 
large  compared  with  both  Vr  and  V2  then  2(^1  H~  ^2) 
be  neglected  and 


QUADRANT  ELECTROMETER  13 

and  therefore  for  a  given  difference  of  potential  between  the 
quadrants  the  deflection  6  will  be  proportional  to  the  potential 
of  the  needle.  It  is  found  however  by  observation  that  in 
most  electrometers  the  deflection  at  first  increases  with  increase 
of  potential  on  the  needle  until  it  reaches  a  maximum  and  then 
decreases  with  further  increase  of  potential.  Recently  G.  W. 
Walker  has  developed  a  theory  in  which  he  accounts  for  this 
maximum  value  for  6  followed  by  a  decrease  by  the  presence 
of  the  air-gap  between  the  quadrants  which  causes  the  capacity 
of  the  needle  to  alter  as  it  moves  past  this  air-gap.  He  obtains 
a  modified  equation,  namely, 


FIG.  8. 


14  APPARATUS   AND   GENERAL    METHODS 

where  b  is  a  constant,  and  assuming  as  before  that  F3  is  large 
compared  with  F±  and  F2  which  is  a  fact  in  the  ordinary 
method  of  using  the  instrument.  From  either  theory  it  fol- 
lows though,  that  for  a  given  potential  of  the  needle,  the 
deflection  is  proportional  to  the  difference  of  potential  between 
the  two  pairs  of  quadrants.  In  most  cases  in  actual  practice 
this  is  the  important  condition  as  a  basis  of  measurement. 

ii.  Dolazalek  Type  of  Electrometer. — The  type  of  elec- 
trometer devised  a  few  years  ago  by  Dolazalek  is,  in  the  opinion 
of  the  author,  the  most  useful  and  satisfactory  instrument  of 
its  kind  made  at  the  present  time  for  the  class  of  work  with 
which  we  have  to  deal,  and  if  an  electrometer  is  to  be  acquired 
for  a  laboratory  this  type  is  most  strongly  recommended.  The 
advantages  of  this  instrument  over  older  forms  consist  chiefly 
in  the  simplicity  of  construction  and  consequent  elimination  of 
troublesome  factors  and  in  the  increased  sensitiveness.  A 
diagram  of  the  instrument  is  shown  in  Fig.  8. 

The  set  of  quadrants  is  small,  usually  only  from  5  to  6  cm. 
in  diameter.  In  the  latest  form  the  quadrants  are  supported 
on  stout  amber  pillars  as  amber  is  an  excellent  insulator.  The 
needle  is  extremely  light  and  is  made  of  silvered  paper  and 
consists  of  two  layers  of  paper  fastened  at  the  outer  edges 


FIG.  9. 

and  spread  apart  at  the  center  to  secure  rigidity.  Broadside  and 
edge-on  views  of  it  are  shown  in  Fig.  9.  Being  so  extremely 
light  and  lying  very  close  to  the  quadrants  the  needle  is  suffi- 


QUADRANT  ELECTROMETER  1 5 

ciently  damped  by  the  air  and  requires  no  other  damping. 
The  conducting  phosphor-bronze  method  of  suspension  with 
permanent  connection  to  the  battery  is  the  most  satisfactory 
to  use.  Usually  a  potential  of  from  80  to  100  volts  on  the 
needle  will  be  found  convenient. 

"\Yith  a  fine  quartz  suspension  the  instrument  may  be  made 
very  sensitive,  as  much  as  10,000  dimensions  per  volt  on  a 
scale  about  1.5  meters  distant  having  been  obtained.  It  is 
however  not  at  all  advisable  to  work  with  the  instrument 
under  such  sensitive  conditions,  for  the  needle  is  very  unstable 
and  will  not  maintain  a  steady  zero  position  and  is  very  easily 
disturbed  by  the  slightest  outside  influence.  For  most  work 
a  sensitiveness  of  anywhere  from  four  to  six  hundred  divi- 
sions per  volt  on  a  scale  about  2  meters  distant  will  be  found 
suitable.  The  sensitiveness  depends  upon  the  thickness  of  the 
suspension  and  the  charge  on  the  needle. 

The  whole  instrument  is  enclosed  by  a  tightly  fitting  brass 
case  with  a  window  through  which  to  observe  the  mirror  on 
the  needle.  . 

12.  Adjustment  of  Lamp  and  Scale. — The  stem  of  the 
needle  should  be  fitted  with  a  small  and  very  light  concave 
mirror,  preferably  not  more  than  6  or  7  mm.  in  diameter, 
having  a  focal  length  of  about  50  cm.  Obtain  the  image  of 
an  incandescent  electric  lamp  filament  on  a  scale  at  a  distance 
of  1.5  or  2  meters  away  from  the  mirror.  Adjust  the  position 
of  the  lamp  so  that  a  sharply  defined  image  of  the  lamp  fila- 
ment is  obtained  in  a  vertical  position  on  the  scale.  Place  the 
lamp  a  little  below  the  level  of  the  mirror  so  that  the  reflected 
beam  of  light  may  pass  directly  over  the  top  of  the  lamp,  caus- 
ing the  plane  of  the  incident  and  reflected  rays  to  be  vertical. 
Place  the  scale  at  right  angles  to  the  beam  of  light  by  carefully 
measuring  equal  distances  from  the  mirror  to  the  ends  of  the 
scale  when  the  image  is  at  the  centre  of  the  scale.  A  white 
millimeter  paper  scale  about  a  meter  long  fastened  to  a  board 
which  may  slide  horizontally  in  grooves  so  as  to  be  adjustable 
to  any  zero  point  will  be  found  convenient.  As  the  image  of 
the  lamp  filament  may  have  a  considerable  width,  select  one 
edge  of  the  image  and  use  it  as  the  line  to  read  by  on  the  scale. 


1 6  APPARATUS   AND   GENERAL    METHODS 

13.  Adjustment  of  Electrometer. — Set  the  electrometer  up 
in  a  dry  atmosphere  on  a  firm  support  such  as  a  slate  slab  rest- 
ing on  a  stone  or  brick  pillar,  so  that  there  may  be  no  vibra- 
tion. Carefully  level  the  instrument  so  that  the  quadrants  are 
level.  Adjust  the  position  of  the  needle  until  it  hangs  sym- 
metrically with  regard  to  the  quadrants,  that  is,  so  that  the  line 
bisecting  the  needle  lengthwise  is  parallel  to  the  air  line  bisect- 
ing the  circle  of  the  quadrants.  After  this  is  adjusted  as  nearly 
as  possible  by  the  eye  test  the  symmetry  by  charging  the  needle, 
having  all  the  quadrants  connected  to  earth.  If  everything  is 
symmetrically  situated  the  zero  of  the  needle  should  not  alter. 
If  the  needle  does  not  remain  at  zero  when  thus  charged  adjust 
it  by  trial  till  it  does.  In  some  instruments  one  of  the  quad- 
rants is  also  adjustable  in  position.  A  further  test  of  the  sym- 
metry should  be  made  as  follows :  One  pair  of  quadrants  being 
to  earth,  charge  the  other  pair  to  a  given  positive  potential  and 
observe  the  deflection,  then  change  the  potential  to  an  equal 
negative  one  and  observe  the  deflection  on  the  opposite  side 
of  the  zero.  These  two  deflections  should  be  exactly  equal  if 
everything  is  symmetrical. 

In  all  instruments  the  quadrants  are  always  connected  in 
pairs  and  each  pair  connected  by  insulated  wires  to  two  insu- 
lated terminals  on  the  outside  of  the  instrument.  The  insula- 
tion of  the  quadrants  and  terminals  should  be  perfect,  as 
defective  insulation  is  fatal  to  accurate  measurements.  Test 
the  insulation  as  follows:  One  pair  of  quadrants  being  con- 
nected to  earth,  disconnect  the  other  pair  from  everything 
else  and  charge  them  up  by  connecting  them  for  an  instant 
to  one  pole  of  a  battery,  the  other  pole  being  to  earth.  Use  a 
small  potential  for  this;  only  a  fraction  of  a  volt  will  be  suffi- 
cient. Having  removed  the  charging  wire  observe  the  reading 
of  the  needle  when  it  comes  to  rest.  If  the  insulation  is  per- 
fect this  reading  should  remain  steady,  but  if  the  insulation 
is  defective  the  needle  will  gradually  return  to  zero,  showing 
that  the  charge  is  leaking  off  the  quadrants.  The  insulation 
should  then  be  removed  and  cleaned  unless  the  leakage  is  so 
small  that  it  will  not  affect  appreciably  the  measurements  to 
be  made. 


ELECTRICAL  SCREENING  I  7 

The  air  inside  the  instrument  ought  to  be  thoroughly  dry. 
If  the  instrument  is  one  in  which  no  condenser  with  sulphuric 
acid  is  used  a  small  quantity  of  drying  material  may  be  intro- 
duced in  a  convenient  place  with  advantage. 

14.  Screening. — There  are  usually  stray  electrostatic  charges 
produced  by  friction  or  other  causes  in  the  neighborhood  of 
the  apparatus  in  use,  especially  in  a  dry  atmosphere.  These 
charges  are  sure  to  cause  serious  electrostatic  disturbances 
either  by  direct  contact  with  the  electrometer  and  connections 
or  by  induction.  Even  the  movement  of  one's  body  near  the 
electrometer  will  often  cause  violent  disturbances  of  the  needle. 
It  is  therefore  absolutely  essential  to  enclose  the  electrometer 
and  all  insulated  parts  of  the  apparatus  and  wire  connections 
within  metal  screens  connected  to  earth  so  that  any  outside 
disturbance  may  not  reach  the  insulated  parts  of  the  apparatus. 
All  parts  which  are  not  required  to  be  insulated  must  be  con- 
nected to  earth.  A  convenient  way  to  do  this  in  most  labora- 
tories is  to  solder  a  fairly  heavy  bare  copper  wire  permanently 
to  a  water  pipe  or  gas  main  which  runs  into  the  earth  and  all 
earth  connections  may  be  made  to  this  wire.  It  is  not  suffi- 
cient to  merely  wrap  this  wire  around  the  water  pipe ;  good 
solder  connection  must  be  made.  If  this  earth  connection 
of  the  different  parts  be  neglected  these  parts  become  charged 
up  and  cause  disturbances  which  are  usually  indicated  by  the 
erratic  action  of  the  electrometer  needle. 

In  setting  up  an  electrometer  for  permanent  use  the  follow- 
ing method  of  screening  will  be  found  most  convenient :  Place 
a  large  sheet  of  metal  about  three  feet  square  on  the  table  and 
set  the  electrometer  on  this.  Make  a  rectangular  cage  of  wire 
gauze  about  three  feet  each  way  enclosed  on  all  sides  but  the. 
bottom.  This  may  be  set  over  the  electrometer  on  the  metal 
sheet.  One  whole  side  of  this  cage  should  be  made  removable 
either  in  the  form  of  a  door  or  otherwise.  The  cage  and  door 
are  made  large  so  that  the  electrometer  may  be  conveniently 
reached  to  make  adjustments,  and  in  addition  several  other 
pieces  of  small  apparatus  such  as  connecting  keys,  etc.,  may  be 
placed  inside  the  screen,  thus  saving  additional  screens. 

3 


iS  APPARATUS   AND   GENERAL    METHODS 

nect  the  whole  cage  permanently  to  earth.  Connections  from 
outside  and  the  light  falling  on  the  mirror  of  the  needle  may 
be  admitted  into  this  screen  through  small  holes  cut  in  the  gauze. 
The  following  method  of  screening  insulated  wires  connect- 
ing separate  pieces  of  apparatus  has  been  found  very  conve- 
nient: Take  a  brass  tube  of  the  required  length  about  2.5  cm.  in 
diameter  and  across  each  end  fix  a  piece  of  clean  stick  sealing 
wax  as  shown  in  Fig.  10.  This  may  be  done  by  heating  the  end 
of  the  tube  slightly  and  pressing  the  sealing  wax  on  to  it.  The 
wire  to  be  screened  is  run  through  the  tube  and  one  end 
fastened  to  the  sealing  wax  by  slightly  heating  the  wire  and 
pressing  it  into  the  wax.  When  this  hardens  the  wire  may  be 
drawn  tightly  at  the  other  end  and  fastened  in  a  similar  way. 
The  wire  remains  tightly  stretched  along  the  axis  of  the  tube. 
The  wire  used  should  be  of  small  diameter  so  as  to  be  flexible 


FIG.  10. 

and  easily  stretched.  For  this  purpose  a  fresh  stick  of  sealing 
wax  should  be  used  and  the  insulating  surface  must  not  be 
handled  by  the  fingers  or  destruction  of  the  insulating  quality 
is  likely  to  result. 

15.  Insulation. — The  question  of  insulation  in  electrostatic 
work  is  quite  a  different  problem  from  what  it  is  in  ordinary 
current  work,  for  what  in  ordinary  current  work  would  be 
perfect  insulation  is  useless  as  such  in  electrostatics.  Since 
the  electrostatic  charges  to  be  measured  are  usually  small 
a  small  leakage  will  cause  quite  a  serious  error.  The  dif- 
ferent so-called  insulators  differ  somewhat  in  quality  for 
electrostatic  uses.  Amber  and  ebonite  are  good  permanent 
insulators  and  possess  the  advantage  of  being  workable  into 
any  shape  in  a  lathe.  A  particular  quality  of  artificial  amber, 
supplied  by  instrument  makers  is  excellent.  Sulphur  is  a 
most  excellent  insulator  for  this  work,  but  has  the  disad- 
vantage of  being  brittle  and  is  apt  to  crack  after  being  cast  in 


INSULATION  19 

large  pieces.  For  small  insulators  it  is  excellent.  In  making 
up  small  sulphur  beads  use  a  clean  piece  of  stick  sulphur  and 
melt  it  very  carefully  in  a  clean  porcelain  dish,  and  just  when 
it  reaches  a  clear  liquid  state  use  it  for  moulding  whatever 
is  required.  If  it  is  heated  beyond  this  stage  it  loses  its  in- 
sulating quality.  Sealing  wax  is  very  excellent  where  no 
strain  is  to  be  put  upon  it.  If  it  is  heated  in  adjusting  it  in 
position  care  must  be  taken  not  to  char  it  as  it  will  lose  its 
insulating  quality  if  burned.  Paraffin  is  a  fair  insulator  when 
pure  but  has  the  serious  disadvantage  of  retaining  very  per- 
sistently any  charge  on  its  surface  as  the  charge  seems  to  pene- 
trate below  the  surface  and  is  removed  with  difficulty.  Glass  is 
not  of  much  value  in  this  class  of  work  as  an  insulator  unless 
coated  with  some  other  insulator,  such  as  paraffin  or  sulphur. 

The  first  general  precaution  in  this  regard  is  that  the  sur- 
face of  the  insulation  must  be  perfectly  clean.  The  outer 
film  of  the  surface  should  be  entirely  removed  by  scraping  or 
otherwise,  so  as  to  leave  a  fresh  surface.  After  the  surface 
has  been  cleaned  it  should  not  be  touched  with  the  fingers,  for 
the  slightest  contamination  destroys  the  insulating  properties. 
This  precaution  is  often  neglected  and  trouble  ensues.  Mois- 
ture and  dust  even  in  small  quantities  on  the  surface  of  in- 
sulation should  be  guarded  against.  The  atmosphere  of  the 
room  where  work  is  done  should  therefore  be  kept  dry.  It 
will  be  necessary  in  many  instances  to  dry  the  air  in  and 
around  the  apparatus  by  means  of  chemical  drying  materials. 

Troublesome  disturbances  often  arise  from  the  surface  of 
the  insulation  acquiring  an  electrostatic  charge.  This  is 
usually  manifest  by  the  erratic  action  of  the  electrometer  needle 
when  the  quadrants  are  insulated.  This  surface  charge  can 
usually  be  removed  by  passing  a  Bunsen  flame  quickly  over  the 
surface.  Another  method  of  removal  is  to  place  a  small  open 
vessel  of  uranium  oxide  close  to  the  charged  surface,  when 
the  ionization  produced  by  the  uranium  will  allow  the  charge 
to  leak  away  through  the  air. 

In  a  complicated  system  of  insulators  it  is  sometimes  rather 
difficult  to  locate  the  exact  position  of  the  leak  due  to  a  de- 


20 


APPARATUS    AND   GENERAL    METHODS 


fective  insulator.  To  do  so  insulate  the  whole  system  and 
charge  it  up  and  observe  the  deflection  of  the  needle  as  ex- 
plained in  §  13  in  connection  with  the  test  of  the  insulation  of 
the  quadrants.  Then  cut  off  the  conductor  in  the  system 
farthest  from  the  electrometer  and  test  again.  If  the  leak 
does  not  appear  now  it  must  have  been  in  the  insulation  of  this 
conductor.  If  it  does  appear  cut  off  the  next  farthest  con- 
ductor and  test  and  continue  this  one  by  one  until  the  leak  is 
located.  Then  clean  or  replace  the  insulation. 

Flames  of  any  kind  should  not  be  allowed  in  the  neighbor- 
hood of  any  object  which  requires  to  be  insulated,  as  the  ioniza- 
tion  produced  by  flames  in  the  air  destroys  the  insulating 
property  of  the  air.  Any  radio-active  substance  in  the  vicinity 
will  cause  the  same  difficulty. 

16.  Electrometer  Keys. — As  any  movement  of  the  observer 
near  the  electrometer  connections  is  apt  to  cause  electrostatic 


FIG.  ii. 


disturbances  and  as  all  the  connections  must  be  carefully  en- 
closed in  earth-connected  screens,  it  is  both  expedient  and  con- 
venient to  place  all  connecting  and  disconnecting  keys  inside 


ELECTROMETER   KEYS  2i 

the  cage  with  the  electrometer  and  work  them  from  a  distance. 
The  keys  shown  in  Fig.  n  are  most  convenient  forms. 

The  key  shown  at  (a)  is  used  for  alternately  connecting  to 
earth  and  insulating  one  pair  of  quadrants  of  the  electrometer 
and  connections.  A  piece  of  sheet  brass  one  sixteenth  inch 
thick  and  an  inch  wide  and  nine  inches  long  is  bent  twice  at 
right  angles,  making  the  upper  part  AB  two  inches  in  length 
and  CD  one  and  a  half  inches.  A  brass  tube  HK  about  J  inch 
inside  diameter  and  2.5  inches  long  is  fitted  and  soldered  at 
right  angles  to  AB  through  a  hole  near  A,  so  that  it  projects  f 
inch  below  AB.  A  brass  rod  RS  about  six  inches  long  and 
pointed  at  5  slides  loosely  through  HK  and  has  two  small  ad- 
justable clamps  Ci  and  C2,  so  that  it  moves  within  defined 
limits.  This  rod  may  be  drawn  up  by  a  cord  attached  to  the 
ring  at  the  top  and  allowed  to  fall  under  its  own  weight. 
This  system  is  fastened  to  a  piece  of  heavy  sheer  metal 
XY  and  the  whole  thing  connected  to  earth.  An  insulated 
metal  mercury  cup  Q  is  placed  so  that  S,  when  lowered, 
will  dip  into  it.  The  electrometer  quadrants  and  connections 
are  connected  with  the  mercury  cup,  and  when  the  rod  RS  dips 
into  Q  they  are  all  connected  to  earth  through  ABC  and  when 
RS  is  raised  they  are  insulated. 

One  precaution  must  be  carefully  observed  here  which  ap- 
plies to  all  cases  where  electric  contact  is  made  between  mer- 
cury and  other  metals,  such  as  brass  and  copper.  The  surface 
of  the  metal  in  contact  with  the  mercury  must  always  be 
amalgamated,  by  dipping  first  in  nitric  acid  and  then  rubbing 
mercury  over  the  surface,  so  as  to  eliminate  contact  difference 
of  potential  effects  at  the  juncture  of  two  dissimilar  metals. 
The  end  of  the  rod  S  and  the  inside  of  the  cup  Q  must  be 
amalgamated.  Otherwise  the  electrometer  needle  will  be  de- 
flected when  the  contact  is  broken,  due  simply  to  the  change  of 
potential  at  the  separation  of  the  two  dissimilar  metals  brass 
and  mercury. 

After  experimenting  for  some  time  with  different  forms 
of  mercury  cups  and  other  contacts  for  5"  the  author  found 
the  form  of  cup  and  insulator  shown  in  the  diagram  the  most 


22  APPARATUS   AND   GENERAL    METHODS 

satisfactory.     Q    is   a   small   brass    cup   about    I    cm.    inside 
diameter  and  I  cm.  deep  and  it  rests  on  a  stick  of  clean  sealing 
wax  about  3  cm.  high.     The  sealing  wax  is  fastened  to  the' 
cup  above  and  the  plate  below  by  slightly  heating  the  metals 
and  sticking  the  wax  to  the  warm  metal. 

Mercury  cups  have  often  been  made  by  boring  a  hole  in  a 
block  of  paraffin  and  filling  with  mercury,  but  they  are  not 
satisfactory  as  the  paraffin  becomes  charged  up  on  the  surface 
by  the  friction  of  the  key  and  mercury  and  otherwise,  and  this 
charge  does  not  disappear  immediately  when  the  cup  is  earthed 
on  account  of  the  retentive  quality  of  the  paraffin.  In  all 
cases  of  this  sort  the  area  of  the  insulating  surface  should  be 
reduced  as  much  as  possible. 

A  slight  modification  (b)  of  the  above  form  of  key  has  been 
found  very  convenient  for  making  contact  between  two  in- 
sulated connections  within  the  electrometer  screen,  such  as  for 
instance  connecting  a  standard  cell  to  the  electrometer  or  con- 
necting a  separate  charged  condenser  with  the  electrometer. 
The  rod  RS,  instead  of  dipping  into  the  mercury  cup,  is  fitted 
tightly  into  a  rod  of  ebonite  /  about  an  inch  or  so  long  and 
into  the  other  end  of  the  ebonite  is  fitted  another  brass  rod  M 
which  may  dip  into  the  mercury  cup.  M  is  thus  insulated  from 
earth  by  /  and  is  connected  to  the  cell  or  other  object  by  a 
thin  flexible  wire  w.  The  rest  of  the  key  is  identical  with  (a). 

These  keys  are  raised  by  means  of  a  cord  which  runs  over  a 
pulley  in  the  ceiling  vertically  above  the  key  and  is  thence 
carried  to  any  desired  position  by  another  pulley  placed  about 
vertically  above  the  scale  where  the  observer  is  situated  when 
taking  readings. 

17.  Standardization  of  Electrometer. — To  convert  the  scale 
readings  into  definite  electrical  units  these  readings  must  be 
standardized  by  observing  the  deflections  produced  when  a 
known  potential  is  applied  to  the  quadrants.  If  the  electrom- 
eter is  not  too  sensitive  one  pole  of  a  standard  Clark  cell  or 
Weston  cell  may  be  applied  directly  to  the  insulated  pair  of 
quadrants  by  means  of  the  key  (&),  Fig.  n,  while  the  other 
pole  is  connected  to  earth.  To  guard  against  accidentally 


STANDARDIZATION  OF  ELECTROMETER 


short-circuiting  the  standard  cell  by  neglecting  to  insulate  the 
quadrants  from  earth  before  connecting  the  cell  to  them  a 
large  resistance  of  about  10,000  ohms  should  be  placed  in 
series  between  the  one  pole  and  the  earth,  so  that  in  case  of 
accident  the  cell  would  not  be  injured.  If  the  deflection  pro- 
duced by  the  cell  is  not  too  great  for  the  length  of  scale  used 
observe  the  deflection  produced  by  the  known  voltage  of  the 
cell.  If  the  deflection  is  too  great  for  the  length  of  scale 
connect  the  cell  through  a  potentiometer  of  large  resistance 
and  tap  off  from  the  potentiometer  a  known  potential  sufficient 
to  give  a  suitable  deflection.  Since  the  deflections  are  propor- 
tional to  the  difference  of  potential  between  the  quadrants -the 
number  of  scale  divisions  per  volt  is  known.  This  value  is 
technically  termed  the  sensitiveness  of  the  instrument.  Any 
other  deflection  on  the  scale  can  therefore  be  converted  into 
volts  by  direct  proportion.  The  sensitiveness  of  course  depends 
upon  the  potential  of  the  needle  and  the  instrument  must  con- 
sequently be  standardized  each  time  observations  are  made  if 
there  is  any  change  of  potential  of  the  needle. 

1 8.  Connection  of  Electrometer  to  Other  Apparatus. — The 
wires  connecting  the  electrometer  to  any  other  apparatus,  on 
which  experiments  are  being  conducted,  must  be  screened  in 
metal  tubes  connected  to  earth  as  already  described,  §  14.  The 
typical  general  arrangement  of  electrometer,  lamp  and  scale, 
key,  screen,  etc.,  is  shown  in  the  diagram,  Fig.  12,  which  is 


FIG.  12. 


24  APPARATUS   AND    GENERAL    METHODS 

self  explanatory.  The  apparatus  to  be  tested  would  be  con- 
nected, by  a  screened  wire  passing  through  the  cage,  to  the 
mercury  cup  in  the  key.  The  metal  tube  screening  this  con- 
necting wire  should  pass  inside  the  main  cage  so  that  no  part 
of  the  wire  is  exposed. 

19.  Determination  of  Capacity  of  Electrometer  and  System. 
—In  making  definite  calculations  from  the  observations  with 
the  electrometer  it  is  often  necessary  to  know  the  capacity  of 
the  electrometer  and  connections.  One  method  of  determin- 
ing this  is  by  the  ordinary  method  of  mixtures. 

Let    C  =  the  capacity  of  electrometer  and  system  connected 

with  it, 
C1  =  the  capacity  of  a  known  standard  condenser. 

Charge  up  the  electrometer  and  connections  by  means  of  a 
battery  to  a  potential  V  and  let  the  deflection  of  the  needle  be  d 
divisions  on  the  scale.  Then  connect  by  means  of  a  key  (b), 
Fig.  u,  the  standard  condenser  in  parallel  with  the  charged 
electrometer,  etc.  The  charge  will  then  be  divided  between  the 
electrometer  system  and  the  standard  condenser.  Let  the  re- 
sulting potential  =  V±  and  the  corresponding  scale  reading 
=  d1-  therefore  CV  =.  (C  -f-  C^)V^  since  the  total  charge  is 
constant. 

S~*  '         T7  J 

Therefore  =_<  =  _!, 


from  which  C  may  be  calculated. 

Another  method  depending  upon  the  very  constant  ionization 
produced  by  uranium  oxide,  and  which  will  be  better  under- 
stood after  some  experiments  on  ionization  have  been  done, 
furnishes  a  very  convenient  means  of  determining  capacity. 
The  arrangement  is  shown  in  Fig.  13.  Two  insulated  metal 
plates  A  and  B,  about  15  cm.  square,  are  enclosed  in  a  metal 
box  connected  to  earth.  One  plate  A  is  connected  to  the  elec- 
trometer ;  the  other  B  is  connected  to  one  pole  of  a  battery  of 
accumulators  of  about  200  volts,  the  other  pole  being  to  earth. 


DETERMINATION   OF   CAPACITY 


25 


On  B  is  sprinkled  a  small  quantity  of  uranium  oxide  which 
produces  a  constant  amount  of  ionization  in  the  gas  between 
A  and  B.  For  a  given  constant  source  of  ionization  and  a 
given  potential  on  B  the  rate  at  which  A  charges  up  is  pro- 
portional to  the  capacity  of  the  system.  Let  C  be  the  capacity 
of  the  electrometer,  ionization  vessel  and  connections  and  let 
d  be  the  number  of  scale  divisions  passed  over  per  second  as 


s 

fA 

« 

ZZ& 

re  AW 

*TH        r"£n 

m 

AX  AT  US 
'ST£D 

A 

1—  1 

B 

\TH 


£ARTM 


FIG.  13. 


A  charges  up,  and  let  the  potential  acquired  by  A  per  second  be 
V.  Now  connect  a  standard  condenser  5"  of  capacity  C\  in 
parallel  by  means  of  the  key  K  and  let  the  number  of  scale 
divisions  per  second  be  dt  and  the  potential  acquired  per  second 
be  V±.  Then,  since  the  charge  acquired  in  each  case  is  the 
same, 


Therefore 


C 


5  * 

V 


I . 
d' 


or 


d-d. 


from  which  C  may  be  calculated  as  before. 

This  method  has  the  advantage  of  convenience  and  even 
reliability,  and  in  addition  has  the  advantage  of  the  capacity 


26  APPARATUS   AND   GENERAL    METHODS 

being  determined  with  the  needle  in  motion  under  conditions 
similar  to  those  under  which  the  actual  test  measurements  are 
made.  It  has  the  slight  disadvantage  however  that  the 
capacity  determined  includes  the  capacity  of  the  ionization 
vessel,  which  cannot  be  eliminated  and  therefore  it  must  be 
kept  constantly  connected  to  the  electrometer  during  all  the 
experimental  measurements.  The  uranium  oxide  however 
must  then  be  removed  and  everything  connected  to  earth  ex- 
cept the  plate  A. 

20.  Typical  Measurement  of  Current  by  Electrometer.— 
As  a  guide  to  the  method  of  measurement  and  calculation  of  a 
current  by  the  electrometer  a  typical  example  will  be  given. 
The  usual  method  of  measurement  is  to  observe  the  time  rate 
at  which  the  electrometer  quadrants  charge  up,  that  is,  the 
quantity  of  electricity  transferred  through  a  gas  per  second  to 
a  conductor  connected  with  the  electrometer. 

Let  q  =  the  number  of  coulombs  of  electricity  received  by 
the  electrometer  quadrants  and  connected  system 
per  second. 

Let  the  capacity  of  the  whole  system  in  microfarads  (in 
which  units  condensers  are  usually  made)  be  C, 
which  will  equal  C/io6  farads. 

Let  V  =•  the  rise  of  potential  of  the  system  per  second  in 
volts. 

£7 
Therefore  q  =  — 6  x  V. 

Therefore  since  the  current  equals  the  number  of  coulombs 
per  second,  the  current  i  will  equal  CF/io6  amperes. 

The  deflection  of  the  electrometer  needle  is  proportional  to 
the  difference  of  potential  between  the  two  pairs  of  quadrants. 

Let  d  —the  number  of  scale  divisions  moved  over  per 
second  as  the  quadrants  acquire  their  potential  V. 

Let  c^  — the  number  of  scale  divisions  corresponding  to 
a  difference  of  potential  of  I  volt  as  determined 
from  a  standard  cell. 


MEASUREMENT  OF   CURRENT  27 

Therefore  V=  ^ ; 

«i 

C       d 
or  i  =  — 6  x  -j  amperes. 

In  practice  the  capacity  of  a  Dolazalek  electrometer  system 
without  any  extra  condenser  is  usually  about  50  electrostatic 
units,  which  we  will  take  as  a  typical  case. 

Therefore          (Twill  =  --       — ^microfarads, 
9  x  io5 

since  i  microfarad  =  9  X  io5  electrostatic  units.  Suppose  that 
di  =  6oo  divisions  and  d=io  divisions  per  sec.;  then  will 

50  i         io 

1  =  9  x  io*  x  io6  x  6ob 

=  9.2  x  io'13  amperes. 

21.  Preliminary    Experiments    with    the    Electrometer. — 

Before  attempting  any  definite  ionization  measurements  with 
the  electrometer  one  should  become  perfectly  familiar  with  the 
instrument  by  a  little  preliminary  manipulation,  (i)  In  the 
first  place  the  electrometer,  its  lamp  and  scale,  keys  and  all 
accessories  should  be  very  carefully  set  up  and  adjusted  as 
already  explained.  It  pays  to  spend  a  little  time  on  the  pre- 
liminary setting  up  as  subsequent  experiments  will  be  per- 
formed with  much  greater  ease  and  accuracy.  (2)  Test  the 
sensitiveness  of  the  instrument  for  various  potentials  on  the 
needle.  (3)  Connect  a  storage  battery  of  a  couple  of  volts 
across  a  potentiometer  and  measure  in  volts  by  the  electrometer 
the  potential  across  different  portions  of  the  potentiometer. 

22.  Electroscopes. — The  gold  leaf  electroscope  was  one  of 
the  earliest  electrical  instruments.     In  its  older  forms  it  was 
used  chiefly  as  a  detector  of  the  presence  of  an  electric  charge 
and  was  not  well  suited  to  accurate  quantitative  measurements. 


28 


APPARATUS   AND   GENERAL    METHODS 


Since  there  has  arisen  of  late  years  the  need  of  a  delicate  and 
accurate  instrument  to  measure  currents  too  small  to  be  meas- 
ured even  by  an  electrometer  the  electroscope  has  reached  a 
high  state  of  perfection.  It  assumes  somewhat  different  forms 

when  applied  to  certain  spe- 
cial cases,  but  the  form  shown 
in  Fig.  14  is  the  general  type 
of  instrument  used  and  will 
serve  for  most  purposes  where 
an  electroscope  is  required. 
AB  is  a  metal  case  which 
may  vary  in  size  according 
to  the  purpose  for  which  it  is 
required,  but  a  convenient 
size  is  one  of  about  a  liter 
capacity.  C  is  a  tightly  fit- 
ting ebonite  plug  about  2.5 
FIG.  14.  cm.  diameter.  A  metal  rod 

D    about     i     mm.    diameter, 

passes  through  the  ebonite,  and  on  the  end  of  it  is  a  piece  of 
amber  or  a  bead  of  sulphur  to  insulate  the  gold  leaf  system. 

In  making  this  sulphur  bead  great  care  must  be  taken  to 
secure  good  insulation.  It  may  be  made  as  follows:  Heat 
very  gently  a  small  quantity  of  clean  stick  sulphur  in  a  clean 
porcelain  evaporating  dish  until  it  melts.  When  it  becomes  a 
clear  liquid  dip  the  end  of  the  rod  D  into  it  and,  withdrawing 
the  rod,  allow  the  sulphur  to  harden.  Repeat  this  continuously 
until  quite  a  large  bead  has  accumulated  on  the  end  of  D. 
Great  care  must  be  taken  in  heating  the  sulphur  to  prevent 
heating  it  too  much  and  it  must  only  be  used  just  at  the  clear 
liquid  stage,  for  if  it  is  heated  too  much  and  goes  beyond  this 
stage  its  insulating  property  deteriorates  very  much,  and  it 
becomes  almost  useless  as  a  perfect  insulator.  Take  another 
flat  metal  rod  H  from  5  to  7  cm.  long  and  form  another  sul- 
phur bead  on  the  end  of  it ;  then  dip  both  these  beads  into  the 
liquid  at  once  and  stick  them  together  so  the  rods  are  in  a 
straight  line  and  let  them  harden.  If  desired  the  surface  of 


ELECTROSCOPE  29 

the  sulphur  may  be  trimmed  off  with  a  sharp  knife,  but  care- 
fully avoid  touching  the  surface  with  the  fingers  and  keep  the 
surface  clean. 

At  £  a  narrow  strip  of  gold  or  aluminium  leaf  from  3  to  4 
cm.  long  and  I  to  2  mm.  wide  is  attached  by  a  touch  of  gum. 
The  leaf  should  be  cut  between  two  sheets  of  paper  and  with 
as  smooth  an  edge  as  possible  so  as  to  give  a  sharp  line  in  the 
reading  microscope.  For  very  sensitive  electroscopes  the  leaf 
should  be  cut  as  narrow  as  possible.  This  may  be  done  by 
pressing  the  edge  of  a  sharp  razor  full  length  flat  against  the 
leaf  between  the  paper  and  carefully  sawing  the  razor  back 
and  forth.  By  this  means  strips  a  small  fraction  of  a  milli- 
meter in  width  can  be  cut. 

To  charge  the  gold  leaf  system  a  wire  F  passing  through 
an  ebonite  plug  and  bent  twice  at  right  angles  may  be  used. 
This  can  be  turned  round  so  that  one  end  touches  H  above  the 
gold  leaf  and  the  charge  may  be  communicated  from  a  battery. 
F  must  then  be  turned  away  and  connected  to  earth.  If  the 
case  AB  is  required  to  be  air-tight  for  any  reason  F  may  be 
replaced  by  a  flexible  piece  of  steel  spring  wire  attached  to  the 
rod  D  as  shown  in  (a)  Fig.  14.  This  wire  may  be  drawn  into 
temporary  contact  with  H  by  a  magnet  from  outside. 

After  the  gold  leaf  system  is  charged  and  when  readings 
are  being  made  every  part  of  the  instrument  but  the  gold 
leaf  system  must  be  carefully  connected  to  earth,  for  if  any 
part  of  the  instrument  is  insulated  and  becomes  charged  in 
any  way  the  gold  leaf  will  be  disturbed,  as  it  is  so  sensitive. 
For  this  reason  glass  or  any  other  insulating  material  should 
never  be  used  as  a  case  for  the  electroscope  unless  it  is  coated 
on  the  inside  by  some  conducting  material  such  as  tin  foil  or 
a  silver  coating  from  a  silvering  solution  or  something  of  that 
nature.  Non-conducting  material  as  a  case  for  an  electroscope 
is  to  be  avoided  wherever  possible.  Neglect  of  this  precaution 
is  often  the  cause  of  serious  disturbing  influences  and  erratic 
movements  of  the  gold  leaf.  For  this  reason  also  the  necessary 
insulating  parts  should  be  made  as  small  as  possible  consistent 
with  good  insulation,  so  that  there  may  be  as  small  a  surface 
as  possible  to  become  charged  up. 


30  APPARATUS   AND   GENERAL    METHODS 

23.  Illumination    of    Gold    Leaf    and    Scale. — Two    small 
windows  should  be  made  in  the  front  and  back  of  the  case 
opposite  each  other  and  in  line  with  the  leaf.     The  openings 
may  be  closed  by  thin  sheets  of  mica  waxed  down  around  the 
edges.     An   incandescent  lamp   or   other   steady   illumination 
should  be  placed  a  little  distance,  a  foot  or  more,  behind  the 
back  window  so  as  to  illuminate  the  field.     Care  must  be  exer- 
cised with  regard  to  this  illumination  as  the  heat  from  it  some- 
times produces  air  currents  in  the  electroscope  and  causes 
erratic  movements  of  the  gold  leaf,  especially  just  after  the 
illumination  is  turned  on.     Where  observations  extend  over 
several  hours  even  at  intervals  it  is  best  to  keep  the  illumina- 
tion turned  on  all  the  time  and  to  allow  a  little  time  to  elapse 
at  the  beginning  after  turning  on  the  light  before  taking  read- 
ings to  allow  the  conditions  to  become  steady. 

24.  Adjustment  of  Reading  Microscope. — The  movements 
of  the  gold  leaf  are  viewed  through  a  reading  microscope  with 
a  micrometer  scale  within  it.     The  usual  method  of  using  the 
electroscope  is  to  charge  up  the  leaf  system  and  then  observe 
the  rate  at  which  this  charge  leaks  away  through  the  surround- 
ing gas  due  to  ionization  in  the  gas  from  any  cause.     This 
rate  of  leak  is  observed  by  noting  the  time  it  takes  the  gold 
leaf  to   pass   over  a   given   number  of   scale   divisions   as   it 
gradually  falls  due  to  loss  of  charge.     The  time  is  taken  by 
means  of  a  stop  watch. 

Set  up  the  microscope  in  front  of  the  window  of  the  electro- 
scope and  carefully  focus  it  on  the  leaf  so  that  a  clear  image 
of  the  leaf  is  seen  on  the  micrometer  scale.  As  the  edge  of 
the  leaf  usually  appears  somewhat  ragged  in  the  microscope 
due  to  magnification  select  a  definite  point  on  the  leaf  to  ob- 
serve and  adjust  the  microscope  so  that  this  point  is  on  the 
scale.  As  any  given  point  on  the  leaf  in  falling  is  really  mov- 
ing in  a  circle  round  E  (Fig.  14)  as  centre  and  not  in  a  hori- 
zontal line  the  micrometer  scale  should  be  tilted  at  an  angle 
to  the  horizontal  so  that  the  leaf  in  losing  its  charge  will  move 
over  as  nearly  as  possible  equal  lengths  of  scale  for  equal 
losses  of  charge.  Even  with  this  adjustment  the  leaf  will  not 


ELECTROSCOPE  3 1 

move  over  the  whole  range  of  its  motion  at  the  same  rate  and 
therefore  when  different  readings  are  being  compared  the 
readings  should  all  be  made  over  the  same  portion  of  the 
scale,  that  is,  the  time  taken  for  the  leaf  to  pass  over  the  dis- 
tance between  the  same  two  fixed  points  on  the  scale  should 
be  observed  in  each  case.  This  precaution  is  essential,  for  if 
the  readings  are  taken  over  different  sections  of  the  scale  dis- 
cordant figures  are  likely  to  result. 

25.  Calibration  of  Electroscope. — To  make  absolute  deter- 
minations of  current  by  the  electroscope  the  scale  divisions 
must  be  standardized  in  terms  of  volts.     To  do  this  connect 
a  battery  of  known  voltage,  say  three  or  four  hundred  volts, 
to  a  potentiometer  of  large   resistance   from   which   definite 
known  potentials  may  be  tapped  off.     Charge  the  gold  leaf 
to  sufficient  potential  to  deflect  it  to  a  point  near  the  beginning 
of  the  portion  of  the  scale  over  which  readings  are  to  be  taken 
and  note  the  reading ;  then  increase  the  potential  by  a  known 
amount  so  as  to  increase  the  reading  to  about  the  upper  limit 
of  the  region  over  which  readings  are  to  be  taken.     Note  this 
reading  and  the  difference  in  the  two  readings  will  therefore 
correspond  to  the  known  increase  of  voltage.     Several  voltages 
between  these  two  extreme  ones  should  also  be  taken  and  the 
readings  noted  to  test  whether  the  readings  over  the  different 
parts  of  the  scale  used  are  proportional  to  the  voltage.     If 
they  are  not  a  careful  calibration  over  the  part  of  the  scale  to 
be  used  should  be  made  and  a  calibration  curve  plotted.     Any 
movement  of  the  leaf  across  the  scale  will  then  correspond  to  a 
known  change  of  potential  of  so  many  volts. 

26.  Capacity. — The   capacity   of   an   electroscope   is   some- 
times required  in  making  absolute  quantitative  measurements. 
This  may  be  determined  by  the  method  of  mixtures  already 
described,  §  19,  in  connection  with  the  electrometer. 

27.  Typical  Measurement  of  Current  by  the  Electroscope. 
— A  sample  measurement  of  a  very  small  current  by  an  elec- 
troscope will  be  given  as  illustration.     If  C  is  the  capacity  of 
the  electroscope  in  farads  and  V  the  loss  of  potential  in  volts 
per  second  then  as  in  the  case  of  the  electrometer  i=CV. 


32  APPARATUS   AND   GENERAL    METHODS 

In  practice  the  capacity  of  a  gold  leaf  system  of  an  average 
sized  electroscope  of  about  1,000  c.c.  in  volume  is  usually  about 
one  electrostatic  unit.  Suppose  the  loss  of  potential  were  10 
volts  per  hour,  which  could  easily  be  measured,  then  the  loss 
per  sec.  would  be  10/3600.  Therefore  the  current  through 
the  gas  would  be 

I  10 

i  =  -       — n  X  — > —  ampere, 
9  x  lo11      3600 

since  9  X  IO11  electrostatic  units  =  I  farad. 
Therefore  i  =  3.08  X   io"15  ampere. 

An  even  smaller  rate  of  leak  of  the  gold  leaf  system  may 
easily  be  read  with  accuracy  so  that  the  electroscope  is  capable 
of  measuring  much  smaller  currents  than  in  the  electrometer. 

28.  Preliminary  Experiments  with  the  Electroscope. — A 
few  preliminary  experiments  should  be  performed  with  the 
electroscope  to  familiarize  oneself  with  its  use. 

1.  Carefully  set  up  an  electroscope  with  its  reading  micro- 
scope and  accessories   and  adjust  them  carefully  as  already 
described. 

2.  Calibrate  the  scale  carefully  over  its  whole  workable  range 
and  plot  a  calibration  curve  with  volts  for  ofdinates  and  scale 
divisions  for  abscissae. 

3.  Measure  several  known  voltages  across  a  potentiometer. 

4.  There  is  always  a  certain  amount  of  natural  electrical 
conductivity    through    the    air   which    is   usually    termed   the 
natural  leak  of  the  electroscope  (see  Chapter  XVI).     Measure 
this   carefully   and   calculate  the   current  through   the   air   in 
amperes. 

Note. — In  using  the  electroscope  to  measure  ionization  cur- 
rents this  natural  leak  of  the  instrument  is  always  present  and 
must  be  corrected  for  as  it  is  included  in  the  total  rate  of  leak 
measured.  It  must  therefore  always  be  measured  as  a  pre- 
liminary experiment  and  subtracted  from  the  total  rate  of  leak 
to  obtain  the  true  current  whose  value  is  required. 


CONDENSERS  33 

29.  Condensers. — Another  class  of  instrument  of  very  fre- 
quent application  in  this  line  of  work  is  the  condenser.  In 
many  cases  the  rate  at  which  the  electrometer  charges  up  is 
far  too  rapid  to  be  read  with  any  degree  of  accuracy  or  even 
at  all.  This  rapid  rate  has  to  be  cut  down  to  readable  dimen- 
sions by  adding  capacity  in  parallel  with  the  electrometer.  An 
adjustable  condenser  of  known  capacities  is  therefore  neces- 
sary. If  one  or  more  standard  subdivided  condensers  are 
available  the  problem  is  solved,  but  as  they  are  expensive  they 
are  not  available  in  most  laboratories  for  more  than  a  very  few 
students. 

(a)  Standard  Condenser. — A  simple  form  of  condenser 
which  may  be  used  as  an  absolute  standard  may  be  easily  made 
in  the  form  shown  in  Fig.  15.  Two  brass  tubes  of  equal 


FIG.  15. 

length,  each  of  uniform  size  throughout  and  one  a  little  smaller 
than  the  other,  are  placed  with  their  axes  coincident.  The 
inner  one  is  supported  and  insulated  by  two  ebonite  supports, 
while  the  outer  one  does  not  necessarily  require  to  be  insulated. 
If  this  is  made  accurately  its  capacity  C  in  electrostatic  units 
may  be  calculated  by  the  ordinary  formula 


b9 

*e  d 

where  /  is  the  length  of  each  of  the  cylinders,  a  the  external 
diameter  of  the  inner  cylinder  and  b  the  internal  diameter  of 
the  outer  one. 

(b)  Sulphur  Condenser. — A  working  condenser  of  small 
capacity  may  be  made  as  follows :  Make  20  or  25  plates  of  brass 
or  zinc,  all  of  the  same  size  and  each  about  100  square  cm.  area 

4 


34  APPARATUS  AND   GENERAL   METHODS 

and  about  0.5  mm.  in  thickness.  Fasten  half  of  the  number 
rigidly  by  one  edge  to  a  brass  rod  at  right  angles  to  their  planes 
so  that  they  are  all  exactly  parallel  to  each  other  and  about  0.5 
cm.  apart.  Do  the  same  with  the  other  half  and  then  place  the 
plates  of  one  set  within  the  spaces  of  the  other  set  so  that  they 
are  equally  spaced  and  parallel.  Fasten  these  two  sets  rigidly 
by  clamps  in  this  position,  being  careful  that  they  are  ac- 
curately spaced  and  place  them  inside  a  vessel  to  act  as  a 
mould  and  fill  the  mould  with  pure  sulphur  melted  to  a  clear 
liquid.  When  it  hardens  remove  the  mould.  The  capacity 
of  this  condenser  will  of  course  have  to  be  measured  by  com- 
parison with  the  standard  condenser,  using  the  electrometer  in 
one  of  the  methods  already  described.  Condensers  of  a  variety 
of  sizes  may  be  made  after  this  pattern. 

(c)  Paraffined  Paper  and  Tinfoil  Condenser. — A  condenser 
of  fairly  large  capacity  in  small  compass  may  be  simply  made 
with   sheets  of  tinfoil  and  paraffined  paper.     Dip   sheets  of 
thin  paper  of  say  a  foot  square  or  more  in  melted  paraffin  so 
they  are  thoroughly  saturated  and  then  allow  to  dry.     Build 
up  a  condenser  with  these  paraffined  paper  sheets  as  dielectric 
between  alternate  sheets  of  thin  tinfoil.     The  dimensions  of 
the  tinfoil  sheets  should  be  from  1.5  to  2  inches  smaller  each 
way  than  the  paper  so  as  to  ensure  good  insulation  at  the  edges. 
Each  tinfoil  sheet  may  be  made  with  a  tongue  projecting  from 
one  edge  to  which  connection  may  be  made.     The  alternate 
sheets   should  be  placed  so  that  the  projecting  tongues  are 
situated  at  opposite  edges,  so  the  sheets  of  each  set  or  any 
portion  of  them  may  be  easily  connected  together.     A  con- 
denser of  quite  a  large  capacity  may  be  made  in  this  form, 
and  besides  being  comparatively  cheap  and  easily  made  it  has 
the  advantage  that  any  portion  of  the  total  number  of  plates 
may  be  used  at  a  time  by  disconnecting  the  remainder  and  con- 
necting them  to  earth.     The  capacity  may  be  calculated  ap- 
proximately by  the  ordinary  formula  for  a  plate  condenser  or 
it  may  be  measured  directly  by  comparison  with  a  standard. 

(d)  Adjustable  Condenser. — If  the  actual  capacity  of  the 
condenser  is  not  required  to  be  known  and  if  small  variations 


HIGH   VACUA  35 

and  fine  adjustments  of  capacity  are  required  for  any  purpose 
a  convenient  form  of  sliding  condenser  may  be  used.  A  set 
of  parallel  metal  plates  20  or  25  cm.  square  and  all  fastened 
rigidly  together  at  the  bottom  about  a  centimeter  apart  are 
placed  in  a  vertical  position.  Another  similar  set  is  hung  from 
two  horizontal  well-insulated  rods  parallel  to  the  plane  of  the 
first  set  on  which  the  second  set  of  plates  may  slide  between 
those  of  the  first  set.  By  sliding  the  upper  insulated  set  of 
plates  between  those  of  the  other  set  the  capacity  may  be  in- 
creased and  quite  fine  adjustments  made. , 

30.  Production  of  High  Vacua. — As  a  great  many  experi- 
ments in  this  work  have  to  do  with  low  pressures  a  few  details 
will  be  given  in  this  regard.  There  are  a  variety  of  vacuum 
pumps  which  may  be  used.  When  rapid  exhaustion  is  re- 
quired a  Fleuss  pump  is  very  suitable  if  a  pressure  not  lower 
than  about  a  millimeter  is  desired.  If  a  pressure  not  lower 
than  the  aqueous  vapor  pressure  is  required  the  ordinary 
vacuum  pump  attached  to  the  water  tap  is  very  convenient  for 
rapid  work.  But  in  the  opinion  of  the  author  the  most  satis- 
factory pump  for  general  use,  especially  in  the  exhaustion  of 
glass  discharge  vessels  of  all  kinds,  such  as  cathode  and  X  ray 
tubes,  is  the  glass  Toepler  mercury  pump  as  described  in 
general  text-books  of  physics. 

The  form  generally  given  in  text-books  is  shown  in  (a) 
Fig.  1 6,  but  a  great  improvement  on  this  is  shown  in  (b)  at 
the  point  A,  where  the  side-tube  T  joins  on  above  the  reser- 
voir R.  When  in  use  the  reservoir  5"  is  first  raised  and  then 
lowered;  the  difference  in  pressure  between  the  vessel  which 
is  being  exhausted  and  the  reservoir  R  forces  the  air  from 
this  vessel  through  the  mercury  in  T  and,  unless  extreme  care 
is  taken  at  the  first  stages  of  exhaustion,  it  drives  the  mercury 
with  considerable  force  at  right  angles  to  the  tube  at  A  and  is 
very  liable  to  break  the  glass  at  a  very  critical  joint.  In  the 
form  in  (b),  however,  the  mercury  does  not  strike  the  tube 
at  right  angles  but  comes  round  the  curve  and  shoots  into  the 
reservoir  Rf  and  the  danger  of  breakage  is  very  greatly  les- 
sened. A  very  high  vacuum  may  be  obtained  with  this  pump.-. 


APPARATUS    AND    GENERAL    METHODS 


There  is  one  slight  drawback  with  this  pump,  namely,  the  pres- 
ence of  mercury  vapor  at  low  pressures.  This  may  however  be 
very  easily  remedied  by  placing  a  quantity  of  gold  leaf  loosely 
rolled  up  in  the  tube  connecting  the  pump  with  the  vessel  to  be 
exhausted.  The  gold  leaf  absorbs  the  mercury  vapor  before 
it  reaches  the  vessel,  which  may  thus  be  kept  quite  free  from 
vapor. 

For  measuring  the  gas  pressures  down  to  about  a  millimeter 
or  two  a  good  manometer  should  be  available  along  with  a 


o 


(a) 


LJ 


FIG.  1 6. 


good  barometer.     For  lower  pressures  a  McLeod  gauge  may 
be  used. 

A  comparatively  rapid  and  simple  method  of  obtaining  a 
low  pressure  depends  upon  the  property  possessed  by  charcoal 


HIGH   VACUA  37 

made  from  cocoanut,  by  virtue  of  which  it  will,  at  the  tempera- 
ture of  liquid  air,  absorb  many  times  its  own  volume  of  air 
and  other  gases.  If  a  quantity  of  this  charcoal  be  placed  in  a 
glass  side  tube  connected  with  the  vessel  to  be  exhausted  and 
this  tube  be  immersed  in  liquid  air  the  air  in  the  vessel  will 
be  rapidly  absorbed  and  in  a  few  minutes  a  pressure  low 
enough  for  the  production  of  cathode  rays  may  be  obtained 
even  without  any  previous  exhaustion.  The  quantity  of  char- 
coal necessary  will  of  course  depend  upon  the  volume  of  gas 
to  be  absorbed.  This  is  an  excellent  method  of  securing  a 
very  high  vacuum  by  first  pumping  most  of  the  air  out  and 
then  causing  the  charcoal  to  absorb  the  remainder.  In  order 
to  maintain  the  high  vacuum  the  charcoal  must  of  course  be 
kept  in  the  liquid  air,  as  when  its  temperature  is  allowed  to 
rise  it  allows  the  absorbed  air  to  escape  again  into  the  vessel. 
A  troublesome  difficulty  arises  in  the  removal  of  the  air 
from  such  vessels  as  cathode  ray  and  Rontgen  ray  tubes  from 
the  fact  that  after  the  low  pressure  has  been  obtained  the  pres- 
sure slowly  rises  slightly  again,  due  to  the  air  occluded  by  the 
walls  and  other  parts  of  the  vessel  gradually  escaping  into  the 
vessel  under  the  diminished  pressure.  This  is  noticeable  if  the 
vessel  be  allowed  to  stand  a  while  after  being  exhausted,  or 
if  the  walls  of  the  vessel  be  slightly  heated,  or  again  in  the 
case  of  an  electric  discharge  tube  of  any  kind  if  the  discharge 
be  caused  to  pass.  The  heating  of  the  vessel  or  the  passage  of 
the  discharge  causes  the  occluded  air  to  escape  from  the  walls 
or  electrodes  of  the  vessel.  In  making  a  permanent  Rontgen 
ray  tube  or  anything  of  that  nature  the  vessel  should  be  first 
exhausted  as  low  as  possible  and  the  discharge  caused  to  pass 
for  some  time,  and  as  the  air  accumulates  from  the  electrodes 
it  may  be  pumped  out  until  a  permanently  high  vacuum  is 
obtained.  Sometimes  the  electric  discharge  has  to  be  main- 
tained for  several  hours  before  a  steady  condition  is  reached. 
The  troublesome  rise  of  pressure  in  a  newly  exhausted  vessel 
after  standing  for  several  hours  may  in  many  cases  not  be  due 
to  any  leakage  in  the  vessel  but  to  this  slow  escape  of  the 
occluded  gases. 


30  APPARATUS   AND   GENERAL   METHODS 

31.  Making  of  Air-tight  Joints. — As  many  experiments  in 
this  class  of  work  require  the  fitting  together  of  separate  parts 
of  the  apparatus  so  as  to  be  gas-tight  a  few  hints  gained  by 
considerable  experience  in  this  regard  may  be  of  use.  In  glass 
vessels  it  is  sometimes  required  to  fit  electrodes  or  more  com- 
plicated systems  so  as  to  be  rigid  and  gas-tight  while  in  place, 
but  in  such  a  way  that  they  may  be  removed  without  destroy- 
ing the  apparatus  as  would  be  the  case  of  they  were  sealed 
through  the  glass.  This  is  very  simply  done  by  the  method 
shown  in  Fig.  17.  Suppose  that  any  object  E  is  to  be  enclosed 
in  a  vessel  A  so  it  may  be  removed  at  any  time  later.  Turn 
over  the  circular  edge  of  A  to  form  a  lip  all  the  way  round. 
On  a  glass  tube  make  a  bulb  B  of  the  same  relative 
size  to  the  mouth  of  A  as  shown  in  the  diagram. 
Through  the  opening  in  the  bulb  pass  the  rod  sup- 
porting E  and  seal  it  in  the  end  C.  Then  B  may 
CX^  be  placed  in  the  mouth  of  A  and  the  joint  made 
/  gas-tight  with  sealing  wax.  In  making  sealing  wax 
joints  on  glass  the  glass  must  first  be  gently  heated 
till  it  is  hot  enough  to  melt  the  sealing  wax  when 
rubbed  over  the  surface.  If  the  sealing  wax  be 
simply  melted  and  dropped  on  the  cold  glass  it  will 
chip  off  and  be  quite  useless.  When  the  two  glass 
A  surfaces  to  be  joined  are  thus  covered  with  a  fairly 
thick  coating  of  wax  gently  heat  both  and  press  them 
together  and  allow  to  harden.  Then  any  holes  may 
be  closed  by  melting  on  more  sealing  wax  and  very  gently 
heating  it  till  it  runs  well  together.  The  wax  should  not  be 
heated  to  the  point  of  ignition.  A  perfectly  gas-tight  and 
rigid  joint  may  be  obtained  in  this  manner.  Glass  and  metal 
may  be  easily  joined  together  in  the  same  way. 

In  the  case  where  two  parts  of  metal  or  other  material  fit 
together  fairly  closely  but  are  not  air-tight  and  any  mechanical 
strain  is  borne  by  the  solid  parts  themselves,  paraffin  is  about 
as  good  a  material  for  tightly  closing  the  joint  as  anything. 
If  the  paraffin  be  carefully  melted  on  a  piece  of  heated  metal 
and  run  around  the  opening  in  a  fairly  thick  coating  the  joint 


AlR-TiGHT  JOINTS  39 

may  be  made  perfectly  air-tight.  This  of  course  is  of  no  use 
if  the  apparatus  is  to  be  heated  above  about  room  temperature. 
There  are  several  varieties  of  soft  wax  which  work  about 
equally  well  for  this  purpose  but  paraffin,  if  clean,  has  the 
additional  advantage  of  being  a  good  insulator. 

Any  metal  vessel  such  as  a  brass  cylinder  into  which  any 
form  of  apparatus  is  to  be  put  and  then  the  cylinder  closed 
and  heated  to  any  temperature  up  to  300°  C.  or  so  may  be 
made  gas-tight  in  the  following  way:  Around  the  opening  to 
be  closed  attach  a  metal  flange  aa  as  shown  in  Fig.  170. 
This  should  be  attached  by  brazing,  as  solder- 
ing will  not  of  course  stand  very  high  tempera-  1 
tures.  Make  a  metal  plate  bb  as  covering  to 
fit  flat  on  the  flange  and  towards  the  outer  edge 
pierce  both  plate  and  flange  with  holes  so  that 
they  may  be  bolted  together.  Draw  a  lead 


wire  down  to  a  diameter  of  a  millimeter  or  two          JTIG  I7a- 
and    place    this    in    a    circle    on    the    flange 
inside  the  circle  of  holes,  having  the  two  ends  which  have  been 
shaved  down  thin  overlapping.    Ordinary  fuse  wire  serves  the 
purpose  admirably.     Place  the  metal  plate  in  position  on  top 
of  the  wire  and  then  tighten  the  bolts,  gradually  flattening  out 
the  wire  till  it  reaches  about  one  third  its  original  thickness. 
If  carefully  done  this  will  make  a  perfectly  tight  joint  which 
will  stand  temperatures  where  no  kind  of  wax  or  paste  would 
be  of  any  use  at  all. 


CHAPTER  III. 
CATHODE   RAYS. 

32.  Some  Properties  of  Cathode  Rays.  Phosphorescent 
Action, — As  seen  in  §  7  the  electric  discharge  at  low  pressure 
causes  a  stream  of  minute  particles  to  issue  normally  in  straight 
lines  from  the  cathode  which  produce  phosphorescence  in  the 
glass  when  they  impinge  upon  it.  These  cathode  rays  produce 
phosphorescence  in  a  variety  of  substances  besides  glass. 
Tubes  containing  substances  showing  this  action  of  cathode 
rays  may  be  obtained  already  prepared  from  almost  any  of  the 
large. firms  which  supply  general  physical  apparatus.  If  such 
tubes  are  available  observe  the  phosphorescence  produced  in 
the  different  substances  when  the  discharge  passes.  If  the 
discharge  tube  is  made  in;  the  laboratory  several  tubes  may  be 
made,  each  containing  in  the  end  remote  from  the  cathode 
(Fig.  5)  a  different  substance  which  phosphoresces  under  the 
action  of  the  rays.  Such  substances  are  different  kinds  of 
glass,  calc-spar,  potassium  or  barium  platino-cyanide,  and 
several  of  the  rare  earths  such  as  yttrium,  thorium,  etc.  This 
phosphorescent  property  is  a  very  useful  one  in  detecting  and 
observing  the  rays. 

Casting  of  Shadow. — The  rays  may  be  stopped  by  an  opaque 
obstacle  placed  in  their  path.  This  may  be  observed  by  means 


FIG.  1 8. 


of  a  tube  similar  to  that  shown  in  Fig.  18,  in  which  a  piece  of 
metal  a  stands  in  the  path  of  the  rays.    The  rays  falling  upon 

4o 


CATHODE  RAYS  41 

a  are  stopped  and  a  distinct  shadow  is  cast  on  the  end  of  the 
tube  at  b.  Such  a  tube  may  be  obtained  from  almost  any  large 
instrument  firm.  This  piece  of  metal  is  usually  hinged  at  the 
lower  point  so  that  it  may  be  dropped  down  out  of  the  path 
of  the  rays. 

With  such  a  tube  allow  the  discharge  to  pass  for  several 
minutes  with  the  cross  in  the  erect  position  and  observe  the 
shadow  cast.  Then  drop  the  cross  out  of  the  way  and  observe 
the  appearance  on  the  end  of  the  tube.  The  portion  which 
was  originally  in  complete  shadow  will  now  appear  to  phos- 
phoresce more  brightly  than  the  surrounding  parts.  This  is 
due  to  the  peculiar  fact  that  the  glass  shows  fatigue  under  the 
action  of  the  rays  and  diminishes  in  brightness  while  the  part 
which  was  originally  in  shadow  has  not  experienced  this  action 
of  the  rays  and  therefore  appears  brighter. 

Heating  Effect. — Make  a  discharge  tube  of  about  20  or  25 
cm.  in  length  as  shown  in  Fig.  19,  in  which  the  cathode  is  con- 
cave and  spherical  in  curvature  and  Nthe  anode  consists  of  a 
piece  of  platinum  about  2  cm.  square  and  from  0.2  to  0.4  mm. 
thick.  This  anode  should  be  placed  at  the  center  of  curvature 


FIG.  19. 

of  the  cathode  so  the  cathode  rays  may  be  concentrated  upon  it. 
After  the  tube  is  pumped  down  to  the  proper  pressure  send  a 
fairly  strong  discharge  through  the  tube  until  the  anode  begins 
to  glow.  This  glow  is  the  result  of  the  anode  being  heated 
by  the  stream  of  cathode  ray  particles  bombarding  it.  The 
platinum  may  thus  be  made  incandescent,  showing  the  marked 
heating  effect  and  energy  of  the  cathode  rays. 

33.  Magnetic  Deflection  of  Cathode  Rays. — A  discharge 
tube  of  the  pattern  shown  in  Fig.  20  will  be  found  very  suit- 
able for  the  following  experiments.  In  a  tube  of  from  30  to 
40  cm.  in  length  and  about  3  or  4  cm.  diameter  place  an  alumin- 


4-2  CATHODE   RAYS 

ium  diaphragm  about  5  mm.  in  thickness  at  the  point  a  at  a 
distance  of  a  couple  of  centimeters  from  the  cathode.  This 
diaphragm  should  be  pierced  by  a  hole  about  I  mm.  in  diam- 


FIG.  20. 

eter.  Fix  in  the  other  end  at  &  a  phosphorescent  screen  of 
barium  platino-cyanide.  A  distinct  beam  of  cathode  rays  will 
emerge  from  the  hole  in  a  and  produce  a  phosphorescent 
spot  on  b. 

Place  the  tube  between  the  poles  of  a  moderately  strong 
electromagnet,  the  strength  of  which  may  be  regulated  to  suit, 
so  that  the  magnetic  field  is  perpendicular  to  the  plane  of  the 
diagram.  Observe  that  the  bright  spot  on  b  will  move  at 
right  angles  to  the  direction  of  the  magnetic  field.  The  direc- 
tion of  motion  will  depend  upon  the  polarity  of  the  electro- 
magnet. Reverse  the  polarity  of  the  magnet  and  observe  the 
spot  move  in  the  opposite  direction.  Determine  which  is  the 
north  and  south  poles  of  the  magnet  in  each  case  and  note 
carefully  the  direction  of  motion  of  the  spot  in  relation  to  the 
direction  of  the  lines  of  force  of  the  magnetic  field.  Note  that 
the  deflection  is  in  the  same  direction  as  would  be  produced  on 
a  negative  charge  of  electricity  moving  from  the  cathode  to 
the  anode. 

34.  Electrostatic  Deflection  of  Cathode  Rays. — Place  two 
metal  plates  of  about  3  cm.  by  10  cm.  on  opposite  sides  of  the 
tube  and  parallel  to  each  other  as  indicated  by  c  and  d  in 
Fig.  20.  Apply  a  steady  potential  difference  to  these  two 
plates  of  from  500  to  700  volts.  This  will  be  best  obtained 
from  a  set  of  three  or  four  hundred  small  accumulators. 
Observe  the  movement  of  the  spot  on  the  screen.  Reverse 


CHARGE    CARRIED    BY    RAYS  43 

the  voltage  and  observe  that  the  spot  moves  in  the  opposite 
direction.  Note  that  the  direction  is  the  same  as  a  negatively 
charged  body  would  move  under  the  action  of  this  electric 
field  in  each  case.  Both  the  magnetic  and  electrostatic  deflec- 
tions indicate  that  the  cathode  rays  are  negatively  charged 
bodies  moving  with  a  high  velocity  from  the  cathode. 

35.  Cathode  Rays  Carry  Negative  Charge.— The  negative 
charge  carried  by  the  cathode  rays  is  probably  their  most  im- 
portant characteristic.  This  property  was  originally  proved 
by  direct  experiment  by  Perrin  and  his  method  was  later  modi- 
fied by  J.  J.  Thomson. 

A  special  form  of  discharge  tube  is  necessary  for  this  ex- 
periment and  is  shown  in  Fig.  21.  This  may  be  made  in  the 
laboratory,  or  any  glass-blowing  firm  will  supply  it  to  order. 


1  TO  PUM? 

EARTH 

FIG.  21. 

CF  is  a  glass  bulb  from  12  to  14  cm.  diameter.  A  is  the 
cathode  and  B  the  anode  which  should  be  connected  to  earth. 
This  anode  consists  of  a  brass  plug  about  I  cm.  long  pierced 
by  a  hole  about  1.5  mm.  diameter  and  fitting  tightly  into  the 
glass  tube  which  may  be  made  a  couple  of  centimeters  in 
diameter.  The  cathode  rays  after  passing  through  the  hole  in 
B  fall  upon  the  wall  of  the  bulb  at  a  point  C  and  produce 
phosphorescence.  Another  side  tube  of  2.5  or  3  cm.  diameter 
which  is  out  of  the  line  of  fire  of  the  rays  contains  two  coaxial 
metal  cylinders.  The  inner  one  D  has  a  narrow  slit  in  the  side 
as  shown  and  is  carefully  insulated  and  connected  by  a  rod 


44  CATHODE  RAYS 

with  an  electrometer.  The  outer  tube  E  has  a  slit  opposite 
that  in  D  and  is  connected  to  earth.  This  shields  D  and  its 
connecting  rod  from  any  stray  electrical  effects. 

Connect  this  discharge  tube  to  the  air-pump  and  carefully 
exhaust  it  until  cathode  rays  appear  when  the  discharge  is 
passing.  The  discharge  may  be  produced  either  by  an  induc- 
tion coil  or  Wimshurst  machine.  The  rays  will  produce  a 
phosphorescent  spot  at  C.  Test  by  means  of  the  electrometer 
whether  D  has  any  charge.  It  will  probably  be  found  that 
there  is  a  very  slight  indication  of  charge  on  D  due  to  a  little 
stray  ionization  getting  into  D ,  but  this  effect  should  be  small. 
Now,  using  a  comparatively  strong  magnet,  bend  the  beam  of 
rays  round  until  they  fall  upon  the  openings  in  E  and  D. 
The  movement  of  the  rays  may  be  followed  by  the  phosphor- 
escence they  produce.  As  soon  as  they  fall  upon  D  a  sudden 
charging  of  the  electrometer  ought  to  be  observed,  showing 
that  D  is  receiving  a  charge.  Bend  the  rays  still  farther  round 
till  they  miss  the  opening  in  D  and  observe  that  the  charging 
up  of  D  ceases.  Test  the  electrometer  for  polarity  by  a  cell 
to  determine  whether  the  charge  received  by  D  is  positive  or 
negative.  The  test  should  show  that  the  charge  is  negative. 
This  shows  that  the  cathode  rays  carry  a  negative  charge. 

Allow  the  rays  to  fall  for  some  length  of  time  on  D,  and 
observe  that  D  continues  to  charge  up  until  it  reaches  a  certain 
maximum  value  and  will  not  charge  up  beyond  that  value  no 
matter  how  long  the  rays  continue.  This  shows  that  when 
'this  state  is  reached  D  is  losing  charge  as  fast  as  it  is  acquired. 
As  will  be  plain  from  subsequent  experiments  this  is  a  result 
of  the  gas  around  D  being  made  conducting  by  being  ionized 
and  thus  allowing  the  charge  to  leak  off  as  fast  as  it  is  acquired 
after  it  reaches  a  certain  value. 

36.  Velocity  and  Ratio  of  the  Charge  to  the  Mass  of  a 
Cathode  Ray  Particle. — Since  the  cathode  rays  consist  of 
particles  carrying  a  negative  charge  and  moving  with  a  high 
velocity  it  ought  to  be  possible  to  measure  this  velocity  experi- 
mentally and  to  determine  the  relation  between  the  mass  of  a 
particle  and  the  charge  which  it  carries.  The  possibility  of 


VELOCITY  OF   RAYS  45 

deflecting   these    rays   by   a   magnetic   and   electrostatic   field 
furnishes  a  means  of  determining  these  quantities. 

A  special  form  of  discharge  tube  will  be  required  for  this 
determination.  It  should  be  very  carefully  made  by  an 
expert  glass  blower  so  that  the  different  parts  are  carefully 
lined  up  and  accurately  situated  relatively  to  one  another.  The 
form  of  the  tube  is  shown  in  Fig.  22.  The  total  length  of  this 
tube  should  be  in  the  neighborhood  of  60  cm.  C  is  a  flat 


FIG.  22. 

cathode  from  which  the  rays  travel  in  straight  lines.  A  and  B 
are  thick  metal  plugs  about  2.5  cm.  in  length  and  5  or  6  cm. 
apart  and  fitting  tightly  in  the  tube  of  about  2.5  cm.  diameter. 
A  forms  the  anode  and  they  are  both  pierced  by  holes  about  a 
millimeter  in  diameter  which  must  be  in  exactly  the  same 
straight  line  so  that  a  very  narrow  beam  of  rays  may  pass 
along  the  axis  of  the  tube  and  fall  upon  a  screen  of  phos- 
phorescent material  at  the  other  end.  If  the  curvature  of  this 
end  of  the  tube  be  small  the  phosphorescent  material  may  be 
placed  directly  on  the  surface  of  the  glass,  but  otherwise  it 
may  be  placed  on  a  flat  transparent  screen  situated  just  inside 
the  end  of  the  tube.  On  this  screen  is  a  vertical  scale  in 
millimeters.  Near  to  B  are  two  aluminium  parallel  plates  D 
and  E  about  4  cm.  wide  and  10  cm.  long  and  from  2  to  2.5  cm. 
apart. 

Exhaust  this  tube  carefully  and  observe  in  a  dark  room  the 
phosphorescent  spot  produced  by  the  rays  on  the  screen.  It  is 
better  to  use  a  Wimshurst  machine  than  an  induction  coil  to 
excite  the  discharge  tube  for  this  experiment  as  the  Wimshurst, 
if  carefully  run,  will  give  a  more  steady  current  through  the 
tube. 


46  CATHODE    RAYS 

Let  v  cm.  per  second  be  the  velocity  of  the  moving  particle, 
m  its  mass  and  e  the  charge  it  carries  in  electrostatic  units. 
Let  the  tube  be  placed  in  a  strong  magnetic  field  so  that  the 
lines  of  force  are  perpendicular  to  the  plane  of  the  diagram. 
The  beam  of  rays  will  be  deflected  in  a  vertical  plane  so  that 
the  spot  on  the  screen  will  move  along  the  vertical  scale  from 
a  to  b.  Let  this  field  be  of  strength  H. 

When  a  field  H  acts  at  right  angles  to  an  electric  current  C 
the  force  acting  at  right  angles  to  the  plane  of  the  field  and  the 
current  is  H  x  C.  Therefore  the  force  acting  along  the  radius 
of  curvature  of  the  path  of  the  particle  tending  to  deflect  the 
moving  charge,  which  is  equivalent  to  a  current  equal  to  ev  is 
equal  to  Hev.  This  must  be  equal  to  the  centrifugal  force 
of  the  moving  particle  acting  outward  along  the  radius  of 
curvature  which  from  dynamics  is  equal  to  mv~/r,  where  r  is 
the  radius  of  curvature. 

mv* 
Therefore  Hev  =  ---  ;  "  «r 


H  and  r  can  both  be  determined  as  will  be  shown  and  therefore 
mv/e  is  known. 

Now  if  a  difference  of  potential  be  established  between  the 
two  plates  D  and  E  a  uniform  electric  field  will  act  on  the 
beam  of  rays,  and  if  it  is  applied  in  the  right  direction  it  will 
tend  to  deflect  the  rays  in  a  direction  opposite  to  the  magnetic 
deflection.  Let  this  electric  field  be  X  in  electrostatic  units; 
then  the  force  deflecting  the  beam  will  be  Xe.  The  magnetic 
field  and  the  electric  field  may  be  adjusted  so  that  the  deflection 
produced  by  one  will  be  just  equal  to  that  produced  by  the 
other,  and  if  they  are  in  opposite  directions  the  one  will  just 
balance  the  other.  Under  these  conditions,  then, 

Xe==Hev;    ^^    ^f 

X 
therefore  V  =  H'  (2) 


DETERMINATION   OF   €/m 


X  and  H  can  both  be  measured  and  therefore  v  is  determined. 
Supplying  this  value  of  v  in  equation  (i)  the  value  of  e/m 
may  be  determined. 

The  magnetic  field  used  to  produce  this  deflection  should 
be  as  uniform  as  possible,  as  the  above  calculation  is  made  on 
that  assumption.     It  may  be  produced  by  means  of  an  elec- 
tromagnet of  a  form  similar  to  that  shown  in  Fig.  23  of  which 
the  faces  of  the  pole  pieces  are  plane  and  about  4  or  5  cm. 
broad  by  about  10  cm.  in  length.     These  pieces  should  be  just 
far  enough  apart  to  allow  the  tube'to  be  placed  between  them. 
Place  the  tube  between  these  poles 
so  that  the  magnetic  field  is  parallel 
to  the  plane  of  the  plates  D  and  E. 
Adjust  the  current  through  the  coil 
of  the  electromagnet  till  a  deflec- 
tion of  the  spot  of  a  few  millime-  FIG.  23< 
ters  is  produced.     Apply  by  means 

of  a  set  of  accumulators  a  steady  voltage  to  the  plates  D  and 
E  in  the  proper  direction  to  oppose  the  magnetic  deflection. 
Adjust  this  voltage  arrd  the  magnetic  field  till  the  spot  returns 
exactly  to  its  zero  position.  Then  measure  H  and  X. 

The  strength  of  field  H  may  be  measured  very  conveniently 
by  means  of  a  ballistic  galvanometer  and  a  small  search  coil 
which  may  be  placed  between  the  poles  of  the  magnet  when 
the  discharge  tube  is  removed.  From  the  ordinary  theory  of 
the  ballistic  galvanometer  and  of  currents  induced  in  a  coil 
of  known  dimensions  when  suddenly  removed  from  a  magnetic 
field,  as  given  in  any  text-book  on  this  subject,  the  number  of 
lines  of  force  per  square  centimeter,  that  is  H,  between  the 
poles  of  the  magnet  may  be  calculated.  This  should  be  care- 
fully determined. 

X  may  be  measured  in  volts  and  then  reduced.  Supply  these 
measurements  in  equation  (2)  and  obtain  v. 

Remove  the  electrostatic  field  and  observe  the  deflection  ab 
on  the  scale.  From  this  and  the  distance  between  a  and  the 
face  of  B  the  value  of  r  may  be  obtained  as  follows :  Since  in 
any  circle  the  square  on  the  tangent  is  equal  to  the  rectangle 


CATHODE    RAYS 


contained  by  the  segments  of  the  secant  from  the  same  point 
and  since  Bb  is  a  very  short  arc  of  a  very  large  circle  to 
which  aB  is  a  tangent  at  B,  as  shown  in  Fig.  24,  therefore  bD 
may  be  taken  as  practically  equal  to  the  diameter  of  the  large 
circle,  and  therefore  we  have 


Therefore 


aB2  =  ab(ab  +  2r) 
aBz 


2r 


ab 


-ad, 


FIG.  24. 


from  which  r  is  determined  by  measuring  aB  and  ab  in  centi- 
meters. Therefore  on  supplying  the 
values  obtained  for  H,  r  and  v  in 
equation  ( i )  the  ratio  e/m  is  obtained. 
By  the  use  of  this  method  the  aver- 
age value  of  v  has  been  found  to  be 
2.8  X  io9  cm.  per  second.  This  value 
is  not  quite  constant,  as  it  varies  some- 
what with  the  fall  of  potential  in  the 
discharge  tube.  The  value  of  e/m 
has  been  determined  a  great  many 
times  and  by  different  methods,  and 
the  latest  determinations  give  the  value  as  1.7  X  io7. 

37.  Comparison  of  e/m  for  the  Cathode  Particle  with  that 
for  the  Electrolytic  Ion. — In  the  conduction  of  electricity 
through  a  solution  the  electrolytic  ions  which  are  set  free  by 
electrolysis  carry  an  electric  charge  and  the  ratio  of  this  charge 
to  the  mass  of  the  ion  may  also  be  determined.  Take  the  case 
of  the  hydrogen  ion  and  let  M  be  its  mass  in  grams  and  E  the 
charge  which  it  carries.  Since  1.0357  X.io~5  is  the  electro- 
chemical equivalent  of  hydrogen  it  requires  one  coulomb  of 
electricity  to  liberate  1.0357  X  IO~5  grams  of  hydrogen  from 
a  solution.  Therefore  it  requires  96550  colombs,  or  9655 
electromagnetic  units  of  electricity,  to  liberate  one  gram  and 
9^55  X  M  units  to  liberate  M  grams  or  one  ion.  Therefore 
the  hydrogen  ion  in  its  migration  through  the  solution  must 
have  carried  a  charge  of  9655  X  M  units,  and  therefore 


LENARD   RAYS  49 


^  and  we  have  £7^  =  9655,  which  is  very 
approximately  io4.  Since  hydrogen  has  the  smallest  atomic 
mass  known  this  ratio  for  hydrogen  is  the  largest  such  ratio 
known  in  electrolysis. 

By  comparison  then  the  value  of  e/m  for  the  cathode  ray 
particle  is  1700  times  the  value  of  E/M  for  the  hydrogen  ion 
or  atom.  In  a  later  chapter  the  value  of  e  will  be  determined 
and  it  will  be  found  to  be  equal  to  the  value  of  E  for  hydrogen. 
It  follows  then  that  the  mass  of  the  cathode  ray  particle  is 
1/1703  of  the  mass  of  the  hydrogen  atom.  The  cathode  ray 
particle  possesses  the  smallest  mass  yet  known  and  it  is  vari- 
ously called  by  the  name  of  negative  "corpuscle,"  negative 
"  ion  "  or  electron. 

38.  Lenard  Rays.  —  It  was  long  considered  impossible  for 
cathode  rays  to  pass  through  any  solid  material.  Hertz  was 
the  first  to  disprove  this  and  he  showed  that  if  the  rays  fell 
upon  very  thin  aluminium  foil  or  gold  leaf  a  distinct  phos- 
phorescence on  the  other  side  of  the  foil  was  produced  which 
could  be  deflected  by  a  magnet.  Later  Lenard  made  a  very 
thorough  investigation  of  this  question.  The  following  experi- 
ments which  may  be  performed  in  the  laboratory  will  illustrate 
the  methods  which  he  employed.  Make  a  discharge  tube  of 
the  form  shown  in  Fig.  25.  T7\  is  a  glass  tube  about  20  or 
25  cm.  in  length  and  from  4  to  5  cm.  in  diameter.  C  is  a  flat 
aluminium  cathode  supported  by  a  stiff  aluminium  wire  and 
this  wire  is  completely  surrounded  by  a  small  glass  tube  ab 
which  is  sealed  at  b  around  a  platinum  connecting  wire  in  the 
usual  way.  This  may  very  easily  be  fitted  into  the  larger  tube 
7T±  by  joining  a  short  tube  d,  into  which  ab  just  fits,  to  the 
larger  tube  as  shown.  The  opening  between  ab  and  d  may  then 
be  closed  by  sealing  wax.  AA  is  the  anode  which  consists  of  a 
metal  cylinder  about  3  or  4  cm.  long  surrounding  ab  and  whose 
support  passes  out  through  a  side  tube  /.  The  end  7\  of  the 
large  tube  should  be  carefully  ground  flat  so  as  to  fit  on  a  plane 
surface.  Close  this  end  by  a  brass  plate  about  I  mm.  thick 
and  seal  it  to  the  tube  with  sealing  wax  or  marine  glue. 
Through  the  center  bore  a  hole  about  1.5  mm.  in  diameter. 
5 


CATHODE    RAYS 


Cover  the  hole  with  a  sheet  of  thin  aluminium   foil  in  the 
neighborhood  of  0.002  mm.  in  thickness  and  carefully  seal  it 


down.  HH±  is  a  glass  tube  about  15  cm.  long  with  the  end  H 
ground  flat  to  fit  the  plate  and  sealed  to  it  by  wax  or  glue. 

Connect  the  plate  P  and  the  anode  to  earth  and  exhaust  the 
tube  TTj.  until  a  powerful  discharge  of  cathode  rays  is  pro- 
duced. Observe  in  a  dark  room  the  phosphorescence  in  HH^ 
around  the  aluminium  window  in  P.  Observe  that  when  the 
air  in  HH^  is  at  atmospheric  pressure  this  phosphorescence 
extends  only  a  short  distance  beyond  the  window.  Now 
gradually  exhaust  the  tube  HH^  and  observe  that  as  the  pres- 
sure is  lowered  the  rays  extend  farther  into  the  tube,  until, 
when  a  very  low  pressure  is  reached,  a  well-defined  beam  of 
rays  extends  along  the  tube.  Bring  a  magnet  near  this  beam 
and  observe  the  deflection  of  the  rays. 

If  a  phosphorescent  screen  with  a  scale  similar  to  that  in  the 
tube  of  Fig.  22  be  placed  in  the  end  Ht  of  the  tube  HH±  and  a 
magnetic  and  electric  field  be  applied  to  the  beam  of  rays  the 
velocity  of  these  rays  and  the  ratio  e/m  may  be  determined  by 
the  same  method  as  was  described  in  §  36  in  the  case  of  cathode 
rays.  Lenard  measured  these  quantities  and  found  that  these 
rays  were  transmitted  with  a  somewhat  higher  velocity  than 
ordinary  cathode  rays,  but  that  the  ratio  e/m  was  the  same  as 
for  cathode  rays.  These  rays  beyond  the  aluminium  window 
act  in  all  respects  like  cathode  rays.  They  are  identical  with 
cathode  rays,  but  since  they  are  produced  outside  the  cathode 
ray  tube  they  are  usually  called  Lenard  rays  to  distinguish 
them  from  those  produced  inside  the  tube. 


CANAL  RAYS  5 1 

39.  Canal  Rays. — Goldstein,  in  working  with  a  highly  ex- 
hausted tube,  found  that  if  he  used  a  perforated  cathode  instead 
of  a  solid  one  luminous  streams  emerged  through  the  holes  in 
the  cathode  in  the  direction  opposite  to  the  cathode  rays. 
These  rays  have  been  called  Canalstrahlen  or  canal  rays.  They 
produce  phosphorescence  and  they  may  be  deflected  by  a 
magnetic  and  an  electric  field,  but  the  deflection  is  much  less 
than  in  the  case  of  cathode  rays,  and  it  requires  extremely 
strong  fields  to  produce  the  deflection.  The  direction  in  which 
they  are  deflected  is  opposite  to  that  for  the  cathode  rays  which 
indicates  that  they  are  positively  charged  particles.  The 
velocity  and  the  ratio  of  e/m  for  these  particles  have  been 
determined.  It  is  found  that  they  travel  with  a  smaller  velocity 
than  that  of  cathode  rays.  The  ratio  e/m  is  not  constant  as  in 
the  case  of  cathode  rays,  but  shows  a  considerable  variation 
under  different  conditions.  The  maximum  value  found  was 
about  io4,  which,  as  we  have  seen,  is  the  ratio  of  E/M  for 
the  hydrogen  ion  in  electrolysis.  This  indicates  that  the  mass 
of  these  positive  ions  is  at  least  of  the  same  order  as  the 
mass  of  the  hydrogen  atom.  The  positive  ion  therefore  ap- 
pears to  be  atomic  in  size  and  is  at  least  about  1700  times  the 
mass  of  the  negative  ion  produced  in  a  gas  at  low  pressure. 

These  rays  may  be  observed  by  using  a  discharge  tube  of  the 
form  shown  in  Fig.  26.  It  may  be  easily  made  from  a  glass 


C L) 


1 

7o  rvMp 

{ 

1     I- 

FIG.  26. 


tube  about  2.5  cm.  in  diameter  and  25  cm.  long.  The  cathode  C 
consists  of  a  flat  aluminium  disk  whose  support  passes  through 
a  side  tube  so  that  it  may  be  out  of  the  way  of  the  rays. 
The  disk  C  is  perforated  by  a  number  of  holes  about  I  to  1.5 
mm.  in  diameter. 


52  CATHODE   RAYS 

Exhaust  this  tube  and  when  the  pressure  is  in  the  neighbor- 
hood of  that  required  for  the  production  of  cathode  rays  pass  a 
discharge  through  it  and  carefully  watch  in  a  darkened  room, 
as  the  pressure  is  gradually  lowered,  for  the  appearance  of  the 
luminous  streams  emerging  from  the  holes  on  the  side  of  the 
cathode  remote  from  the  anode.  Observe  the  phosphorescence 
produced  on  the  glass.  Apply  a  strong  magnetic  field  and  also 
an  electrostatic  field  to  the  rays  and  note  the  deflection  which 
results  in  each  case  and  the  direction  of  this  deflection. 


CHAPTER  IV. 
RONTGEN    RAYS.      (DESCRIPTIVE.) 

40.  Origin    of    Rontgen    Rays.  —  The    negatively    charged 
cathode  ray  particle  travelling  with  such  a  high  velocity  must 
possess  considerable  kinetic  energy.     Sir  J.  J.  Thomson  has 
shown  mathematically  that  when   a   rapidly  moving  electric 
charge  is  suddenly  brought  to  rest  an  electromagnetic  disturb- 
ance is  produced  in  the  surrounding  medium  and  it  travels 
outward  from  the  suddenly  arrested  particle.     This  condition 
is  fulfilled  when  a  cathode  ray  particle  is  suddenly  stopped  by 
striking  against  a  solid  body.     In  the  year  1895  Rontgen  ob- 
served, in  working  with  an  ordinary  cathode  ray  tube,  that 
some  sort  of  radiation  was  produced  outside  the  tube.     Phos- 
phorescent bodies  placed  outside  the  tube  were  strongly  excited 
and  a  photographic  plate  in  the  neighborhood  became  black- 
ened.    These  radiations  differ  in  many  ways  as  we  shall  see 
from  cathode  rays  and  have  been  called  Rontgen  rays  after 
their  discoverer.    The  name  first  applied  to  them  was  X  rays 
and  this  name  is  still  commonly  used.    They  travel  in  straight 
lines  with  very  high  velocity.    This  velocity  of  propagation  has 
been  measured  by  Marx  and   found  to  be  the  same  as  the 
velocity  of  light,  namely  3  X  io10  cm.  per  second. 

41.  Rontgen  Ray  Focus  Tube. — For  purposes  of  experi- 
mental study  and  the  practical  application  of  Rontgen  rays 
they  are  produced  by  means  of  a  particular  form  of  discharge 
tube  which  is  usually  called  a  focus  tube.     This  tube  takes 
several  modified  forms,  all  of  which  however  conform  to  the 
same  general  principle,  which  is  shown  diagrammatically  in  its 
simple  form  in  Fig.  27.    This  simple  type  of  tube  will  serve  the 
purposes  of  all  the  experiments  described  in  this  chapter  in 
which  no  quantitative  measurements  are  required.    If  however 
the  automatic  form  which  will  be  described  in  the  next  chapter 
is  available  it  will  serve  the  purpose  even  better  for  these  experi- 

53 


54 


RONTGEN   RAYS 


ments.  AB  is  a  large  glass  bulb  anywhere  from  15  to  20 
cm.  in  diameter  with  the  two  electrodes  a  and  b.  The  cathode 
a  consists  of  a  spherical  concave  piece  of  metal  usually  alumin- 
ium. The  cathode  rays  proceed  normally  from  the  surface  of 
a  and  on  account  of  its  spherical  shape  are  brought  to  a  focus 
at  the  point  c  on  the  anode  b.  This  anode  in  its  simplest  form 
consists  of  a  flat  platinum  plate  placed  at  an  angle  of  45°  to 


FIG.  27. 

the  axis  of  a  and  so  that  the  centre  of  b  is  at  the  point  c.  The 
cathode  rays  are  thus  brought  to  a  focus  at  the  centre  of  the 
anode,  and  hence  the  name  focus  tube.  The  electromagnetic 
pulses  or  Rontgen  rays  therefore  have  their  origin  at  the 
anode  b  and  travel  outward  in  all  directions. 

To  generate  the  rays  the  electrodes  are  connected  to  the 
terminals  of  the  secondary  of  an  induction  coil  or  to  a  Wims- 
hurst  machine.  The  discharge  must  of  course  be  sent  through 
the  tube  in  the  right  direction  so  that  a  is  the  cathode.  This 
is  easily  determined  by  the  appearance  of  the  discharge,  for 
when  the  direction  is  correct  the  half  of  the  bulb  towards  a 
cut  off  by  the  plane  of  b  will  be  clearly  defined  by  the  phos- 
phorescence produced  by  the  Rontgen  rays  falling  upon 
the  glass  in  that  half  of  the  bulb  while  if  the  discharge  is  in 
the  reverse  direction  the  phosphorescent  illumination  will  be 
very  irregularly  distributed. 

The  following  experiments  should  be  set  up  and  performed 
in  a  room  which  may  be  completely  darkened  so  as  to  facilitate 
the  observation  of  the  phosphorescent  and  photographic  action. 


PHOSPHORESCENT    ACTION  55 

42.  Phosphorescent  Action  of  Rontgen  Rays.— One  of  the 
earliest  observed  properties  of  Rontgen  rays  was  their  phos- 
phorescent action  on  certain  substances.  This  is  easily  ob- 
served by  the  phosphorescence  produced  in  the  glass  by  the 
rays  falling  upon  the  inside  of  the  bulb  as  mentioned  above. 
This  phosphorescence  may  be  observed  in  a  great  variety  of 
solid  substances  such  as  the  double  sulphate  of  potassium  or 
uranium,  crystals  of  willimite,  platino-cyanide  of  barium  and 
quite  a  number  of  compounds  of  the  alkali  metals.  Obtain 
specimens  of  as  many  of  these  substances  as  possible  and  allow 
the  Rontgen  rays  to  fall  upon  them  in  a  darkened  room  and 
observe  the  luminescence  produced  in  each  case.  Note  the 
differences  in  color  and  intensity  of  the  phosphorescence  in 
the  various  substances. 

A  screen  made  of  one  of  these  substances  will  be  found  very 
useful  and  almost  essential  in  many  qualitative  experiments  on 
Rontgen  rays  for  detecting  the  presence  of  the  rays  and  noting 
differences  in  intensity,  etc;  Such  a  screen  may  be  made  by 
taking  a  thin  sheet  of  white  bristol-board  30  cm.  square  and 
dusting  a  uniform  and  fairly  thick  coating  of  fine  crystals  of 
platino-cyanide  of  barium  over  the  surface  which  has  been 
made  adhesive  by  a  thin  coating  of  paste.  This  should  be 
mounted  in  a  frame.  Such  screens  may  be  obtained  from  any 
scientific  instrument  maker  who  deals  in  Rontgen  ray  apparatus. 

43.  Penetrating  Power  of  Rontgen  Rays. — Rontgen  rays 
differ  in  a  very  marked  degree  from  cathode  rays  in  the  fact 
that  they  are  able  to  penetrate  bodies  of  considerable  thickness, 
while  we  have  seen  that  cathode  rays  can  not.  Their  penetra- 
ting power,  as  well  as  some  of  their  other  properties,  depends 
upon  the  conditions  existing  within  the  Rontgen  ray  bulb. 
With  a  very  low  pressure  within  the  tube,  and  consequently  a 
large  potential  difference  between  the  electrodes,  the  rays  pro-  j 
duced  possess  considerable  energy  and  are  very  penetrating, 
being  capable  of  going  through,  several  centimeters  of  wood 
and  even  several  millimeters  of  a  dense  substance  like  lead. 
Such  rays  are  usually  called  "  hard  rays  "  and  the  bulb  from 
which  they  are  produced  is  often  termed  a  "  hard  "  bulb.  In 


56  RONTGEN   RAYS 

the  case  of  a  higher  pressure  and  consequent  smaller  difference 
of  potential  the  rays  are  less  penetrating  and  are  called  "  soft 
rays."  A  Rontgen  ray  bulb  of  the  simple  type  described  in 
§  41  will  usually  become  "  hard  "  after  being  used  for  a  con- 
siderable time,  owing  to  the  fact  that  at  these  low  pressures 
the  long-continued  passage  of  the  discharge  seems  to  drive  the 
gas  into  the  walls  of  the  bulb  and  thus  lowers  the  pressure. 
The  gas  may  be  driven  out  of  the  walls  again  by  very  carefully 
warming  up  the  bulb  slightly  from  outside. 

If  two  bulbs  are  available,  one  a  "hard"  and  the  other  a 
"  soft "  one,  compare  approximately  the  relative  penetrating 
power  of  the  rays  from  the  two  bulbs  as  follows :  Allow  the 
rays  from  each  of  the  bulbs  in  turn  to  fall  upon  the  fluorescent 
screen  and  note  the  intensity  of  illumination  in  each  case.  Now 
place  in  the  path  of  the  rays  a  sheet  of  wood  of  one  or  two 
centimeters  in  thickness  and  observe  by  means  of  the  screen 
that  the  intensity  is  cut  down  more  by  the  wood  in  the  case  of 
the  soft  rays  than  in  the  case  of  the  hard  rays.  This  may  also 
be  tried  with  many  other  substances  such  as  thin  sheets  of 
aluminium,  brass  or  lead. 

Different  substances  absorb  rays  of  any  particular  type  to 
a  different  degree.  Generally  speaking  the  denser  substances 
produce  the  greater  absorption.  Metals  absorb  the  rays  more 
than  such  materials  as  wood  and  glass,  and  even  the  metals 
differ  widely  among  themselves  in  this  respect.  Aluminium, 
for  instance,  allows  a  much  greater  proportion  of  rays  to  pass 
through  it  than  does  the  same  thickness  of  lead. 

Using  a  bulb  of  medium  degree  of  "  hardness "  test  the 
absorbing  power  of  different  substances  as  follows :  Procure 
specimens  of  different  materials  such  as  aluminium,  brass, 
zinc,  lead,  wood,  glass,  cardboard,  mica,  etc.,  in  the  form  of 
sheets  about  a  millimeter  or  two  in  thickness  and  about  3 
cm.  square.  These  specimens  should  all  be  of  the  same  thick- 
ness. Arrange  four  or  five  of  them  side  by  side  in  the  same 
plane  by  means  of  a  frame  or  otherwise  and  place  them  in  the 
path  of  the  rays  between  the  bulb  and  the  phosphorescent 
screen,  so  that  the  rays  fall  perpendicularly  upon  and  pass 


ABSORPTION  OF  RAYS  57 

through  all  the  specimens  simultaneously.  Observe  the  differ- 
ence in  intensity  of  the  rays  as  shown  by  the  screen  after 
passing  through  each  of  the  specimens.  Repeat  this  for  all 
the  specimens  at  hand  and  note  carefully  the  difference  in 
their  power  of  absorption. 

The  amount  of  absorption  produced  in  a  given  type  of  rays 
depends  of  course  upon  the  thickness  of  the  absorbing  material. 
Procure  several  specimens  of  sheet  aluminium  varying  in  thick- 
ness from  about  .1  mm.  to  5  or  6  mm.  Arrange  these  as  in 
the  previous  experiment  so  that  the  rays  fall  upon  them  simul- 
taneously, and  observe  the  absorption  produced  by  the  different 
thicknesses  of  the  same  material.  Repeat  this  with  a  set  of 
specimens  of  sheet  lead  and  also  any  other  substance  available. 

This  difference  in  the  absorbing  power  of  different  sub- 
stances is  well  illustrated  in  the  case  of  parts  of  the  human 
body.  Place  the  hand  or  arm  close  up  against  the  phosphor- 
escent screen  between  the  screen  and  the  Rontgen  ray  bulb. 
Observe  the  comparatively  dim  outline  of  the  flesh  and  the 
well-defined  outline  of  the  bones,  which  is  due  to  the  fact  that 
the  flesh  absorbs  the  rays  only  to  a  small  extent  while  the  bones 
absorb  them  much  more.  The  latter  thus  casts  a  much  deeper 
shadow  than  the  former.  It  is  by  this  means  that  any  foreign 
substance,  such  as  a  bullet,  may  be  located  within  the  body  by 
means  of  the  Rontgen  rays,  as  such  a  substance  will  cast  a 
deeper  shadow  than  the  surrounding  parts  of  the  body. 

44.  Use  of  Lead  as  a  Screen  from  the  Rays. — It  will  be 
observed  in  the  above  experiments  that  lead  absorbs  the  rays 
to  a  greater  extent  than  any  of  the  other  substances.  This 
great  absorbing  power  of  lead  serves  a  very  useful  purpose  in 
screening  anything  from  the  action  of  the  rays.  It  is  usually 
of  advantage  and  very  often  necessary  to  work  with  a  well 
defined  beam  of  rays.  Since  the  Rontgen  ray  bulb  gives  out 
rays  extending  over  a  large  area  it  becomes  necessary,  in  order 
to  obtain  a  well  defined  beam,  to  screen  off  a  large  proportion 
of  the  rays  and  use  only  those  travelling  in  a  given  direction. 
This  is  done  by  placing  in  front  of  the  bulb  a  large  thick  sheet 
of  lead  with  a  well  defined  hole  of  the  proper  size  cut  in  the 


58  RONTGEN   RAYS 

sheet  exactly  opposite  the  anode  of  the  bulb.  Only  the  rays 
which  emerge  from  this  hole  are  available  for  observation  and 
the  extent  of  this  beam  is  regulated  by  the  size  and  shape  of 
the  hole.  Before  proceeding  farther  with  any  experiments  on 
Rontgen  rays  a  permanent  screen  should  be  set  up  in  a  con- 
venient place  where  it  will  not  require  to  be  moved. 

Make  a  strong  wooden  box  at  least  3  or  4  feet  square.  If 
space  will  allow,  an  even  larger  one  will  be  found  convenient. 
Make  one  whole  side  of  the  box  to  open  on  hinges  as  a  door. 
Carefully  cover  the  box  on  all  six  sides  with  sheet  lead  about 
•J  inch  thick,  being  careful  that  there  are  no  openings  at  the 
joints  of  the  lead.  Set  this  box  up  beside  the  table  on  which 
the  rest  of  the  apparatus  is  to  be  arranged.  In  the  side  of  the 
box  facing  the  table  cut,  at  a  convenient  height,  a  hole  about 
8  cm.  square.  On  the  inside  of  the  box  opposite  the  opening 
set  up  the  Rontgen  ray  tube,  carefully  placing  it  so  that  the 
anode  faces  the  opening  and  the  axis  of  the  tube  is  horizontal 
and  parallel  to  the  face  of  the  box.  This  may  be  done  con- 
veniently by  placing  two  wooden  brackets  on  the  wall  of  the 
box,  one  on  either  side  of  the  opening  and  placing  on  each  of 
these  an  insulating  block  of  paraffin  cut  out  to  fit  the  tubes 
A  and  B  (Fig.  27),  which  rest  on  these  blocks.  When  care- 
fully adjusted  firmly  fix  the  tube  to  these  blocks  by  running  a 
little  melted  paraffin  around  the  place  of  contact.  Care  must 
be  taken  in  doing  this  so  as  not  to  crack  the  tube  by  the 
hot  paraffin. 

Place  inside  this  box  also  the  induction  coil  or  Wimshurst 
machine  and  all  their  connections  by  which  the  bulb  is  to  be 
run.  Make  connections  from  the  induction  coil  to  the  X  ray 
bulb  by  means  of  very  fine  double  covered  wires,  not  larger 
than  about  No.  32,  as  heavy  wires  are  unnecessary  and  are 
apt  to  put  a  strain  on  the  different  parts  of  the  bulb.  Be  very 
careful  that  these  wires  do  not  come  in  contact  with  the  glass 
of  any  part  of  the  tube,  for  if  they  do  the  glass  is  apt  to  be 
punctured  by  a  spark.  Besides  serving  as  a  screen  to  control 
the  beam  of  Rontgen  rays  this  lead-covered  box  serves  to 
screen  off  from  the  testing  and  measuring  apparatus  all  electro- 


PHOTOGRAPHIC  ACTION  59 

static  disturbances  which  might  be  caused  by  the  induction  coil 
and  connections.  All  the  different  sections  of  this  lead  cover- 
ing should  be  carefully  soldered  together  and  connected  to 
earth.  This  screening  and  earth  connection  is  very  important 
to  insure  favorable  conditions  for  working.  A  neglect  of  this 
is  very  often  the  cause  of  trouble  in  this  class  of  work. 

In  order  to  secure  a  beam  of  rays  of  any  desired  shape  or 
area  of  cross  section  it  will  be  found  convenient  to  cut  a  sheet 
of  lead  of  the  same  thickness  as  used  in  covering  the  box  and 
about  25  cm.  square,  and  arrange  on  the  face  of  the  box  a 
pair  of  grooves  into  which  this  sheet  may  slide  so  as  to  oner 
the  opening  in  the  box.  In  this  sheet  a  hole  of  the  desired  size 
and  shape  may  be  cut.  A  number  of  such  interchangeable 
screens  may  be  made  to  suit  the  different  requirements  in 
each  case.  These  screens  should  fit  closely  to  the  face  of  the 
box  so  that  no  stray  rays  may  escape  around  the  sides. 

45.  Photographic  Action  of  Rontgen  Rays. — When  Rontgen 
rays  fall  upon  a  photographic  plate  they  produce  an  effect  ex- 
actly similar  to  that  of  light.  The  effect  produced  depends 
upon  the  intensity  of  the  rays  and  the  length  of  time  of  ex- 
posure. Consequently  if  the  intensity  has  been  diminished  by 
passing  the  rays  through  an  absorbing  material  before  reaching 
the  plate  the  effect  on  the  plate  will  be  diminished.  The  differ- 
ence in  the  absorbing  power  of  different  materials  thus  enables 
us  to  make  Rontgen  ray  photographs.  For  instance,  photo- 
graphs of  the  interior  of  different  parts  of  the  human  body 
may  be  made  as  shadows  if  these  were  thrown  on  the  phos- 
phorescent screen  as  in  §  43.  Rontgen  ray  photographs  differ 
from  light  photographs  in  the  fact  that  the  latter  are  produced 
by  reflection  of  the  light  from  the  object  photographed,  while 
Rontgen  ray  photographs  are  produced  by  the  rays  after  pass- 
ing directly  through  the  object. 

Wrap  up  a  photographic  plate  in  black  paper  so  as  to  ex- 
clude any  light.  This  of  course  does  not  prevent  the  action 
of  Rontgen  rays,  as  they  will  penetrate  the  paper.  Place  this 
enclosed  plate  a  foot  or  two  in  front  of  the  X  ray  bulb.  Place 
the  hand  close  to  the  plate  between  the  plate  and  the  bulb. 


60  RONTGEN    RAYS 

Allow  the  rays  to  act  for  15  or  20  seconds.  This  time  will  de- 
pend greatly  upon  the  strength  of  the  rays  and  will  have  to  be 
tested  by  a  preliminary  trial  for  the  particular  bulb  used. 
Develop  the  plate  in  the  ordinary  way  and  observe  the  im- 
pression produced. 

On  a  thin  board  about  3  or  4  mm.  thick  fasten  various  ob- 
jects, such  as  discs  of  metal  or  coins  of  different  kinds  and 
different  thicknesses  and  objects  of  various  shapes.  Place  this 
set  of  objects  in  the  path  of  the  rays  and  take  a  photograph 
of  it.  Note  carefully  the  difference  in  the  absorption  by  the 
different  objects  as  shown  by  the  difference  in  the  intensity  of 
the  shadows  cast. 

46.  Conductivity  of  Gases  Produced  by  Rontgen  Rays.— 
Probably  the  most  striking  property  of  Rontgen  rays  is  their 
power  to  cause  gases  to  become  conductors  of  electricity.  As 
before  mentioned,  gases  under  normal  conditions  of  tempera- 
ture and  pressure  and  under  ordinary  voltage  are  almost  com- 
plete non-conductors  of  electricity.  If  a  well  insulated  body 
such  as  the  leaves  of  a  gold  leaf  electroscope  be  charged  up  in 
thoroughly  dry  air  the  charge  will  be  retained  for  many  hours. 
There  may  be  an  extremely  slow  diminution  of  the  charge  on 
the  gold  leaves  due  in  part  to  the  want  of  perfect  insulation 
and  partly  due  to  a  very  small  leakage  through  the  air.  If  how- 
ever a  beam  of  Rontgen  rays  be  allowed  to  pass  through  the 
gas  surrounding  the  leaves  they  will  immediately  lose  their 
charge  and  collapse,  showing  that  the  charge  must  have  leaked 
away  through  the  air. 

Set  up  an  electroscope,  of  the  form  described  in  §  22,  Fig. 
14,  at  a  distance  of  25  or  30  cm.  in  front  of  the  window  of 
the  lead  box  containing  the  Rontgen  ray  bulb.  Charge  up  the 
leaves  by  storage  cells  to  a  fairly  high  positive  potential,  and  if 
the  insulation  is  good  the  leakage  of  the  charge  should  be  ex- 
tremely small.  Start  the  X  ray  bulb  and  allow  the  rays  to 
fall  upon  the  air  in  the  electroscope.  Observe  the  sudden 
collapse  of  the  leaves. 

Now  by  means  of  a  lead  screen  without  any  opening  in  it 
placed  over  the  window  cut  down  the  intensity  of  the  rays  to 


CONDUCTIVITY   PRODUCED   BY   RAYS 


6l 


a  small  fraction  of  their  original  intensity  and  also  adjust  the 
position  of  the  electroscope  so  that  a  well  defined  beam  of 
rays  passes  through  only  the  lower  portion  of  the  electroscope 
as  far  away  from  the  leaves  as  possible.  Recharge  the  leaves 
and  start  the  rays  again,  and  if  the  intensity  has  been  cut  down 
sufficiently  the  leaves  should  collapse  at  a  very  much  slower 
rate  than  before.  Charge  up  the  leaves  with  a  negative  charge 
and  repeat  the  experiment.  Observe  that  the  discharge  takes 
place  just  as  before  and  at  just  the  same  rate  as  in  the  case 
of  the  positive  charge. 

These  experiments  show  that  the  air  has  become  conducting 
under  the  influence  of  the  rays  and  discharges  electricity  of 
either  sign  with  equal  facility,  and  the  conductivity  depends  to 
some  extent  at  least  on  the  intensity  of  the  rays.  By  inter- 
posing screens  of  different  thicknesses  the  dependence  of  the 
conductivity  on  the  intensity  of  the  rays  may  be  noted  by 
observing  that  the  less  the  intensity  of  the  rays  the  slower 
the  rate  of  leak  shown  by  the  leaves. 

47.  Transportation  and  Persistence  of  Conductivity. — Ar- 
range a  scheme  of  apparatus  as  shown  in  Fig.  28.  AB  is  a 


thin  brass  tube  about  12  cm.  long  and  4  or  5  cm.  in  diameter 
placed  as  shown  in  front  of  the  window  of  the  Rontgen  ray 
enclosure.  It  is  joined  by  a  temporary  joint  of  large  rubber 


62  RONTGEN    RAYS 

tubing  or  other  convenient  means  to  a  brass  tube  CD  about  30 
or  40  cm.  long  and  3  cm.  diameter.  This  is  joined  at  D  by  a 
similar  temporary  joint  to  the  tube  leading  into  the  electro- 
scope E.  Connect  the  metal  parts  all  to  earth.  A  current  of 
air  may  be  slowly  drawn  through  the  whole  system  entering  at 
A  and  leaving  at  K  by  an  aspirator  attached  to  the  outlet  K. 

Charge  the  leaves  of  the  electroscope.  Start  a  slow  current 
of  air  through  the  system  and  note  that  the  leaves  still  retain 
their  charge.  Now  start  the  X  ray  bulb  with  the  current  of 
air  still  flowing  and  observe  that  the  leaves  immediately  begin 
to  lose  their  charge.  This  indicates  that  the  conductivity  im- 
parted to  the  air  in  AB  may  be  conveyed  by  the  current  of  air 
to  the  electroscope  at  a  considerable  distance  away  and  that 
it  lasts  long  enough  at  least  to  be  carried  that  far.  Stop  the 
rays  and  the  current  of  air  and  recharge  the  electroscope. 
Start  the  X  ray  bulb  again  without  any  current  of  air  flowing 
through  the  system  and  observe  that  there  is  now  no  leakage 
of  the  charge  from  the  leaves.  This  shows  that  it  requires 
an  air  current  or  some  such  means  to  transport  the  con- 
ductivity from  where  it  is  produced  in  AB  to  the  electroscope. 

Again  recharge  the  electroscope  and  run  the  bulb  for  five  or 
ten  seconds  without  any  current  of  air  passing.  At  the  end  of 
that  time  stop  the  bulb  and  after  two  or  three  seconds  start  the 
current  of  air  and  observe  the  slow  discharge  of  the  leaves. 
Repeat  this  but  after  stopping  the  bulb  wait  for  a  slightly 
longer  interval  before  starting  the  air  current  and  note  that  the 
rate  of  discharge  of  the  leaves  is  not  quite  so  rapid.  Repeat 
this  several  times,  each  time  waiting  a  longer  interval  after 
stopping  the  rays  before  starting  the  air  current  and  observe 
the  gradual  diminution  of  the  rate  of  leak  until  finally  if  the 
interval  is  long  enough  no  leak  takes  place  at  all.  These  ex- 
periments indicate  that  the  conductivity  imparted  to  the  air  by 
the  rays  persists  for  a  short  time  after  the  rays  have  ceased. 
It  does  not  last  indefinitely  but  gradually  disappears. 

48.  Removal  of  Conductivity. — Between  B  and  C  at  the 
joint  BC  insert  a  glass  bulb  a  filled  with  cotton  wool,  not  too 
closely  packed.  Start  the  current  of  air  and  also  the  Rontgen 
rays  and  observe  the  effect  on  the  electroscope.  It  should  show 


REMOVAL  OF   CONDUCTIVITY  63 

no  leakage  of  the  charge  from  the  leaves,  showing  that  the  air  / 
in  passing  through  the  cotton  wool  loses  its  conductivity. 

Remove  the  bulb  a  and  substitute  for  it  a  wash  bottle  b 
partially  rilled  wtih  water  and  repeat  the  last  experiment. 
Again  there  should  be  no  discharge  of  the  electroscope,  indi- 
cating as  before  that  the  air  loses  its  conductivity  by  bubbling 
through  -water. 

Remove  the  wash  bottle  b  and  also  the  tube  CD  and  in  its 
place  substitute  the  brass  tube  c,  which  has  about  the  same 
dimensions  as  CD.  Along  the  central  axis  of  this  tube  there 
is  a  stiff  wire  supported  and  insulated  by  an  ebonite  plug. 
When  this  tube  is  in  place  it  should  be  insulated  from  both 
AB  and  the  electroscope.  Connect  the  central  wire  to  one 
pole  of  a  battery  of  small  accumulators  and  the  tube  to  the 
other  pole,  so  that  there  is  a  field  of  about  150  volts  between 
the  wire  and  the  tube.  Now  start  the  Rontgen  rays  and  also 
the  current  of  air  and  observe  whether  there  is  any  leakage 
from  the  gold  leaves.  They  should  show  no  leakage.  Discon- 
nect the  battery  from  the  wire  and  the  tube  and  connect  them 
to  earth  while  the  Rontgen  rays  and  the  air  current  are  still 
running  and  observe  that  the  leaves  immediately  begin  to  lose 
their  charge.  Put  the  electric  field  on  to  the  tube  and  wire 
once  more  but  in  the  reverse  direction  to  what  it  was  before 
and  observe  that  the  discharge  in  the  electroscope  ceases.  The 
conductivity  of  the  air  is  thus  removed  by  passing  through  a 
strong  electric  field. 

49.  These  experiments  show  that  when  Rontgen  rays  pass  ^ 
through  air  it  becomes  a.  conductor  of  electricity  and  this  con-  \ 
ductivity  imparted  to  the  air  by  the  rays  persists  for  a  short  I 
time  after  the  rays  cease  to  act  on  the  air,  but  gradually  dis-  / 
appears.     While  it  lasts  it  may  be  transported  from  one  point 
to  another  along  with  the  air.     This  conductivity  must  be  due 
to  something  mixed  with  the  air,   for  it  is  removed  by  the 
passage  through  the  cotton  wool  and  the  water,  and  its  removal 
by  the  electric  field  also  indicates  that  whatever  is  mixed  with 
the  air  to  produce  this  conductivity  must  be  charged.     In  the 
following  chapter  we  shall  discuss  the  full  significance  of  these 
phenomena, 


CHAPTER  V. 
RONTGEN    RAYS.       (QUANTITATIVE    MEASUREMENTS.) 

50.  In   this   chapter  we  will   investigate  the  properties   of 
Rontgen  rays  more  in  detail,  especially  the  property  of  im- 
parting conductivity  to  gases,  and  discuss  the  methods  of  mak- 
ing quantitative  and  more  precise  measurements  on  the  rays. 

51.  Automatic  Focus  Tube. — The  nature  of  the  rays  and 
the  effects  which  they  produce  depend  to  a  great  extent  upon 
the  conditions  existing  within  the  X  ray  bulb.    The  amount  of 
conductivity  produced,    for   instance,   depends   upon   whether 
the  rays  are  "  hard  "  or  "  soft."     In  making  definite  quanti- 
tative measurements  any  variation  in  the  nature  of  the  rays  is 
therefore  fatal  to  any  attempts  at  accuracy. 

The  chief  source  of  difficulty  in  the  use  of  Rontgen  ray 
bulbs  is  the  tendency  for  the  pressure  within  the  bulb  to 
change,  owing  to  the  passage  of  the  discharge  through  it. 
When  the  discharge  is  continued  for  some  time  the  pressure 
becomes  less  as  the  gas  seems  to  be  driven  into  the  walls  and 
other  parts  of  the  tube.  The  rays  then  become  more  penetra- 
ting. Also  by  the  bombardment  of  the  platinum  anode  by  the 
cathode  rays  it  becomes  heated  and  this  heating  of  the  anode 
liberates  the  occluded  gas,  which  increases  the  pressure.  This 
tends  to  soften  the  rays.  These  two  sources  of  change  do  not 
counterbalance  each  other,  and  consequently  the  simple  form 
of  bulb  described  in  §  41  is  irregular  in  its  action  as  regards  the 
type  of  rays  which  it  gives  out  and  is  consequently  not  suited 
to  quantitative  measurements.  Various  methods  have  been 
tried  to  overcome  this  difficulty,  but  the  most  successful  ones 
are  those  which  have  resulted  in  the  automatic  regulating  tube. 
One  form  of  this  is  shown  in  Fig.  29,  in  which  AB  is  the  main 
bulb  of  the  usual  form.  To  this  is  attached  a  small  side  tube  ab, 
with  two  electrodes,  in  which  is  placed  in  some  cases  a  little 
powdered  caustic  potash  while  in  others  one  of  the  electrodes 

64 


AUTOMATIC    FOCUS   TUBE  65 

has  mica  sheets  attached.  The  anode  of  this  small  tube  is 
attached  to  the  anode  of  the  main  bulb,  while  the  cathode  has 
a  wire  W  attached,  and  between  this  wire  and  the  main 
cathode  is  a  spark  gap  H.  When,  by  the  continued  passage  of 
the  discharge  through  the  main  bulb,  the  pressure  becomes  less 
and  the  resistance  between  the  main  electrodes  consequently 
greater  the  discharge  will  then  pass  across  the  spark  gap,  if  it 
is  short  enough  to  make  its  resistance  sufficiently  small,  and 
through  the  small  tube  ab.  The  heat  of  the  discharge  through 
ab  will  liberate  vapor  from  the  caustic  potash  or  the  mica 


FIG.  29. 

which  will  raise  the  pressure  in  the  whole  system  and  lower 
the  resistance  and  allow  the  discharge  to  pass  through  the  main 
bulb  again.  It  will  continue  to  pass  through  AB  until  the 
pressure  becomes  too  low  again,  when  it  will  once  more  pass 
through  ab.  The  pressure  is  thus  automatically  regulated. 
The  longer  the  spark  gap  H  the  lower  will  the  pressure  in  AB 
become  before  the  discharge  will  pass  across  H  and  through 
ab.  Therefore  by  adjusting  the  length  of  the  gap  H  the  bulb 
may  be  made  to  work  at  any  desired  pressure  within  certain 
limits.  The  longer  the  spark  gap  the  "harder"  will  be  the 
6 


66 


RONTGEN    RAYS 


rays  produced.     This  type  of  bulb  is  very  regular  in  its  action 
and  gives  very  satisfactory  results.* 

A  somewhat  modified  and  improved  type  on  the  same  prin- 
ciple is  shown  in  Fig.  30.  In  this  a  side  tube  K,  containing  a 
chemical  which  gives  off  vapor  when  heated  and  absorbs  it 


FIG.  30. 

again  when  cooled,  is  directly  connected  to  the  main  bulb.  It 
is  surrounded  by  another  tube  R  which  is  exhausted  to  a  low 
Crookes'  vacuum.  In  this  form  the  pressure  in  the  main  bulb 
is  very  low  to  start  with  and  it  has  therefore  a  very,  high 
resistance,  and  at  the  start  the  discharge  will  pass  over  the 
path  of  least  resistance  across  the  spark  gap  and  through  the 
tube  R  which  is  at  a  low  vacuum.  The  bulb  K  is  directly 
opposite  the  cathode  S  in  this  tube  and  the  bombardment  by 
the  cathode  rays  will  heat  K  and  liberate  vapor  from  the 
chemical  contained  in  it.  This  will  continue  till  sufficient 
vapor  has  been  liberated  to  raise  the  pressure  in  the  main 
bulb  so  that  the  resistance  is  low  enough  to  allow  the  discharge 
to  pass  through  it  instead  of  through  R.  When  the  discharge 
through  R  thus  ceases  the  chemical  in  K  will  cool  down  and 
absorb  some  of  the  vapor  which  will  lower  the  pressure  in  the 
main  tube  again.  This  will  be  adjusted  again  by  the  discharge 
passing  through  R  once  more  and  heating  K  and  liberating 
more  vapor.  This  side  tube  attachment  thus  automatically 

*  This  type  of  bulb  may  be  obtained  from  various  manufacturers  of 
X  ray  apparatus. 


MANIPULATION    OF    RONTGEN    RAY    BULB  67 


regulates  the  pressure  in  the  main  bulk.  The  pressure  at  which 
the  tube  works  will  depend  upon  the  resistance  across  the 
spark  gap  and  through  the  side  tube,  that  is  on  the  length  of 
the  spark  gap.  The  working  pressure  may  thus  be  regulated 
by  adjusting  the  length  of  the  spark  gap. 

The  difficulty  caused  by  the  heating  of  the  anode  by  the 
impact  of  the  cathode  rays  is  usually  best  overcome  by  making 
the  anode  of  a  large  piece  of  platinum  so  that  there  is  a  large 
mass  to  heat  and  consequently  less  rise  of  temperature.  The 
anode  is  also  sometimes  supported  by  a  copper  stem  as  the 
copper  conducts  the  heat  away  more  rapidly.  In  some  forms 
there  is  a  water-cooling  arrangement  attached  to  the  anode  but 
this  is  not  very  satisfactory. 

52.  Setting  up  and  Manipulation  of  Rontgen  Ray  Bulb.— 
In  starting  to  use  an  X  ray  bulb  without  any  previous  experi- 
ence great  care  should  be  exercised  as  it  presents  certain  condi- 
tions which  are  not  met  with  in  connection  with  common 
electrical  apparatus.  Although  a  number  of  rules  can  be  laid 
down  for  the  general  use  of  an  X  ray  tube,  still  rules  can  not 
be  given  to  cover  every  contingency  which  may  arise  and  a 
complete  knowledge  of  the  action  of  X  .ray  bulbs  can  only  be 
gained  by  experience.  A  few  general  hints  in  this  regard  may 
be  of  value. 

Set  up  carefully  the  automatic  focus  tube  in  the  lead  box 
as  explained  in  §  44.  Avoid  any  strain  on  the  bulb.  Use 
fine  double  covered  wire,  not  larger  than  about  No.  32,  to 
connect  the  electrodes  of  the  bulb  to  the  terminals  of  the 
induction  coil,  as  heavy  wires  are  apt  to  cause  a  strain  on  the 
bulb  which  is  liable  to  result  in  a  crack.  Do  not  under  any 
circumstances  allow  these  wires  to  touch  the  glass  parts  of 
the  tube  or  to  come  any  nearer  to  the  glass  than  is  really 
necessary,  for  a  spark  is  liable  to  pass  from  the  wire  to  the 
glass  and  cause  a  puncture  of  the  glass  and  ruin  the  tube. 
Connect  the  negative  terminal  of  the  induction  coil  or  static 
machine  to  the  cathode  of  the  bulb  and  the  positive  terminal  to 
the  anode  of  the  main  bulb  and  automatic  regulator.  Careful 
attention  should  be  given  to  this  to  ensure  that  the  discharge 


68  RONTGEN    RAYS 

passes  in  the  right  direction  through  the  bulb.  When  the  di< 
charge  is  in  the  right  direction  the  following  observations  will 
aid  in  making  certain  of  it:  (i)  When  the  main  bulb  lights 
up  the  half  of  the  bulb  opposite  the  face  of  the  anode  should 
be  uniformly  phosphorescent  while  the  half  behind  the  anode 
should  be  comparatively  dark.  If  the  discharge  is  in  the 
wrong  direction  the  illumination  will  be  irregular.  (2)  A 
shadow  of  the  anode  should  be  cast  by  the  cathode  rays  on  the 
bulb  on  the  side  directly  opposite  the  cathode.  If  the  dis- 
charge is  in  the  wrong  direction  this  will  not  appear.  (3)  A 
tube  running  correctly  will  cast  a  well-defined  shadow  of  any 
object  on  the  fluorescent  screen,  while  if  the  discharge  is  in 
the  wrong  direction  the  illumination  on  the  screen  will  be 
faint  and  the  shadows  indistinct. 

If  the  current  is  sent  through  the  bulb  in  the  wrong  direction 
it  causes  platinum  to  be  given  off  from  the  platinum  anode, 
which  is  deposited  on  the  walls  of  the  tube  and  blackens  the 
tube.  When  an  induction  coil  is  used  to  drive  the  bulb  there 
is  always  a  certain  amount  of  reversal  current  which  is  in- 
jurious to  the  bulb.  This  may  be  eliminated  to  a  great  extent 
by  placing  a  spark  gap  between  one  of  the  terminals  of  the  coil 
and  the  electrode  of  the  tube  instead  of  connecting  both 
terminals  directly  to  the  tube.  The  length  of  this  spark  gap 
can  be  adjusted  by  trial  so  that  the  spark  passes  without 
difficulty. 

The  type  of  automatic  make  and  break  used  in  connection 
with  the  induction  coil  is  of  great  importance  in  securing  uni- 
formity of  action  of  the  Rontgen  ray  bulb.  The  ordinary  form 
of  spring  hammer  brake  attached  to  coils  is  not  at  all  suitable, 
especially  for  large  coils,  for  the  contacts  gradually  fuse  and 
their  action  does  not  remain  uniform.  A  Wehnelt  interrupter, 
or  motor-driven  rotary  mercury  interrupter,  or  some  such  uni- 
formly running  type  gives  much  more  steady  action  in  the  bulb 
and  is  much  more  satisfactory. 

Start  the  bulb  up  carefully  with  a  spark  gap  of  from  2  to 
5  cm.  After  it  is  started  this  may  be  regulated  to  any  desired 
length,  depending  upon  the  type  of  rays  required.  Knowledge 


HINTS   ON    MAKING   MEASUREMENTS  69 

of  this  can  only  be  gained  by  experience  and  definite  rules 
cannot  be  laid  down  for  definite  lengths  of  spark  gap. 

To  keep  a  bulb  in  good  condition  never  run  it  continuously 
for  any  considerable  length  of  time  as  the  parts  are  apt  to 
become  heated  and  conditions  change.  When  making  quan- 
titative measurements  it  is  best  to  run  it  not  more  than  from 
twenty  to  forty  seconds  without  a  stop  unless  the  experiment 
really  demands  a  longer  run. 

53.  General  Hints  on  Making  Measurements. — Notwith- 
standing the  great  improvements  in  Rontgen  ray  bulbs  and 
their  adjustments  there  is  still  apt  to  be  considerable  want  of 
uniformity  in  the  results  produced  unless  certain  precautions 
are  taken  in  the  use  of  the  bulb.  When  taking  a  series  of  quan- 
titative measurements  on  the  conductivity  produced  by  Ront- 
gen rays  the  first  precaution  to  be  observed  is  to  run  the  bulb 
regularly,  that  is,  run  it  for  equal  times  and  allow  equal  inter- 
vals of  rest  between  the  runs.  The  bulb  does  not  always  start 
up  at  full  strength  immediately  on  starting,  therefore  it  should 
be  run  for  a  preliminary  five  seconds  or  so  before  starting  to 
take  any  reading  on  the  electrometer  or  other  measuring 
instrument. 

In  making  measurements  a  single  reading  or  observation 
can  never  be  relied  on  alone,  as  might  be  done  in  some  other 
electrical  measurements,  on  account  of  the  want  of  perfect 
constancy  of  the  rays.  The  readings  should  always  be  re- 
peated two  or  three  times  at  least  and  in  some  cases  more,  and 
an  average  of  the  readings  made.  Judgment  will  have  to  be 
used  as  to  the  number  to  be  taken  according  to  the  agreement 
shown  among  the  readings. 

In  making  comparative  measurements  with  the  electrometer 
the  readings  should,  as  far  as  possible,  be  made  over  the  same 
portion  of  the  scale  to  avoid  inaccuracies  in  the  scale  and  also 
errors  due  to  the  difference  of  angle  at  which  the  beam  of  light 
falls  upon  the  scale  at  different  points. 

When  a  series  of  measurements  are  being  taken  with  the 
electrometer  its  sensitiveness  should  be  tested  at  intervals 
during  the  measurements  to  ensure  that  it  is  not  changing, 


?d  RONTGEN   RAYS 

or  if  it  is  changing  to  furnish  a  means  of  correcting  for  the 
change  and  reducing  all  the  readings  to  the  same  basis. 

The  electrometer  should  be  set  up  ready  for  use  in  the 
manner  described  in  Chapter  II  in  a  convenient  permanent 
position  near  by  the  lead  box  containing  the  Rontgen  ray  bulb, 
so  that  it  may  be  connected  to  any  apparatus  set  up  in  front  of 
this  box. 

54.  Production  of  Current  Through  the  Air  by  Rontgen 
Rays. — Cut  two  plates  of  aluminium  about  15  cm.  square  and 
set  them  up  on  edge  on  clean  paraffin  blocks  so  that  they  stand 
vertical.  Place  these  plates  parallel  to  each  other  about  6  or  8 
cm.  apart  and  8  or  10  cm.  in  front  of  the  window  of  the  Ront- 
gen ray  enclosure  so  that  a  beam  of  rays  from  the  bulb  5*  will 
pass  between  them  as  shown  in  Fig.  31.  Over  the  window  place 


EARTH 


EARTH 


FIG.  31. 


a  thick  lead  screen  with  a  rectangular  hole  cut  in  it  about' 1.5 
cm.  broad  and  6  cm.  high  so  that  this  hole  is  directly  opposite 
the  center  of  the  anode  of  the  bulb.  Arrange  the  plates  P  and 
Q  symmetrically  with  regard  to  this  opening.  These  dimen- 
sions are  only  given  as  a  guide,  but  the  exact  width  of  the  hole 
and  the  distance  apart  of  the  plates  and  their  distance  from  the 
hole  must  be  carefully  arranged  so  that  the  cone  of  rays  from 
the  bulb  will  pass  between  the  plates  without  touching  their 
surface.  This  can  be  tested  experimentally  by  holding  the  fluo- 
rescent screen  just  in  front  of  the  plates  and  noting  the  width 
of  the  illumination  on  the  screen  which  should  be  just  a  little 
less  than  the  distance  between  the  plates. 


CONDUCTIVITY    PRODUCED    BY    RONTGEN    RAYS  71 

To  one  plate  P  connect  the  positive  pole  A  of  a  battery  of 
small  accumulators  of  20  or  30  volts,  while  the  negative  pole 
B  is  connected  to  earth.  Connect  the  other  plate  Q  to  one  pair 
of  quadrants  of  the  electrometer  through  a  screening  tube 
(§  14),  the  other  pair  of  quadrants  being  connected  to  earth. 
Connect  in  parallel  with  Q  an  adjustable  condenser  C  as  indi- 
cated. Also  make  a  connection  to  earth  through  the  special 
electrometer  earthing  key  K  (§  16)  which  is  worked  by  a 
cord  at  a  distance. 

Close  the  earthing  key  K,  adjust  the  condenser  to  a  capacity 
of  about  0.5  microfarad  and  start  the  bulb.  After  running 
for  two  or  three  seconds  open  the  key  K.  Note  that  the 
electrometer  needle  immediately  begins  to  indicate  that  the 
quadrants  connected  with  Q  are  receiving  a  charge.  If  the 
movement  of  the  needle  is  too  rapid  increase  the  capacity 
of  the  condenser,  or  if  too  slow  decrease  it.  Observe  that 
this  charging  up  continues  as  long  as  the  rays  continue  to  act. 
Stop  the  rays  and  the  charging  up  will  cease.  Earth  the 
plate  Q  again  through  K  and  reverse  the  connections  of  the 
battery  so  that  P  is  connected  to  the  negative  pole  B  and  A 
-to  earth.  Repeat  the  last  experiment  and  note  that  the  needle 
indicates  that  Q  receives  a  charge  of  opposite  sign  as  the  move- 
ment is  in  the  opposite  direction.  Stop  the  rays  and  close  K 
and  then  test  the  electrometer  with  a  standard  cell  as  to  the 
sign  of  the  charge  indicated  by  the  movement  of  the  needle  in 
either  direction  (§  17).  The  test  should  show  that  in  the 
foregoing  experiment  when  P  was  connected  to  the  positive 
pole  of  the  battery  while  the  rays  were  acting  the  plate  Q 
received  a  positive  charge,  while  when  P  was  at  a  negative 
potential  Q  received  a  negative  charge. 

After  making  a  number  of  preliminary  trials  of  this  nature 
carefully  regulate  the  sensitiveness  of  the  electrometer  needle 
by  adjusting  the  potential  on  it  and  also  adjust  the  capacity  C 
until  the  electrometer  needle  shows  a  movement  of  about  five 
scale  divisions  per  second.  This  can  also  be  regulated  to  some 
extent  by  adjusting  the  intensity  of  the  rays  by  placing  a  metal 
screen  in  front  of  the  window  to  cut  down  the  intensity.  After 


72  RONTGEN    RAYS 

a  convenient  rate  of  movement  of  the  needle  is  obtained  the  rate 
at  which  the  plate  Q  charges  up  can  be  measured,  as  the  rate 
of  charging  up  is  proportional  to  the  rate  of  movement  of  the 
needle,  that  is  to  the  number  of  scale  divisions  passed  over  per 
second.  This  rate  may  be  measured  with  a  stop  watch.  Hav- 
ing made  this  adjustment  repeat  the  above  experiments  and 
observe  carefully  by  a  stop  watch  the  time  taken  for  the  spot 
of  light  to  move  over  a  given  distance  on  the  scale.  Take 
several  readings  first  in  one  direction  and  then  reverse  the 
connections  of  the  battery  and  take  several  in  the  opposite 
direction.  Take  the  average  in  each  case  and  the  same  average 
reading  should  be  found  for  the  two  directions.  Make  a 
number  of  such  observations  so  as  to  become  perfectly  familiar 
with  the  method. 

These  experiments  indicate  that  the  rays  in  some  way  cause 
a  transference  of  electricity  from  the  air  to  the  plate  Q  and 
the  sign  of  the  electric  charge  given  to  Q  depends  upon  the 
sign  of  P.  The  quantity  of  electricity  transferred  per  second 
is  the  same  whether  it  is  positive  or  negative.  There  must  be, 
in  other  words,  a  current  of  electricity  through  the  air  between 
P  and  Q  and  the  current  is  of  the  same  magnitude  whether  P 
is  positive  or  negative  with  regard  to  Q.  The  direction  of  the 
current  depends  upon  the  sign  of  P  with  regard  to  Q. 

55.  Variation  of  Current  with  Voltage. — Connect  the  plate 
P  to  a  potential  of  only  2  or  3  volts  and  measure  the  current 
through  the  air  between  the  plates  as  above,  that  is,  measure 
the  rate  per  second  at  which  Q  receives  a  charge  as  indicated 
by  the  number  of  scale  divisions  moved  over  per  second.  In- 
crease the  potential  of  P  by  a  volt  or  two  and  again  measure 
the  current.  Still  further  increase  the  potential  and  determine 
the  current  produced.  Repeat  this  for  gradually  increasing 
voltages  and  it  will  be  found  that  the  current  rises  with  each 
increment  of  voltage  until  finally  a  stage  will  be  reached  at  \ 
which  the  current  will  no  longer  increase  even  with  a  large 
addition  of  voltage.  In  making  these  observations  take  at 
least  two  or  three  readings  at  each  voltage  and  take  the  mean 
as  the  reading  at  that  voltage.  This  is  necessary  on  account 


SATURATION    CURRENT  73 

of  the  slight  variations  in  the  rays  given  out  which  cause 
variations  in  the  current. 

Plot  this  series  of  readings  on  a  curve  having  for  abscissae 
the  voltages  applied  to  P,  and  for  ordinates  the  corresponding 
currents  observed  in  each  case.  Since  the  current  is  propor- 
tional to  the  number  of  scale  divisions  per  second  the  latter 
may  be  plotted  for  the  current.  This  curve  showing  the  rela- 
tion between  the  voltage  and  the  current  will  assume  the  form 
shown  in  Fig.  32.  It  will  be  seen  that  for  small  voltages  the 
current  obeys  Ohm's  law,  but  soon  begins  to  fall  off  and  finally 
reaches  a  constant  maximum  value  even  for  a  large  increase 
in  voltage.  This  characteristic  curve  is  usually  called  a  satura- 
tion curve  on  account  of  its  similarity  in  form  to  the  satura- 
tion curve  in  the  magnetization 
in  iron.  The  current  corre- 
sponding to  the  flat  part  of  the 
curve  is  called  the  saturation  "g 
current.  When  the  current 
through  a  gas  between  two  elec- 
trodes is  spoken  of  the  satura-  E.MF. 
tion  current  is  meant  unless  it  is  FlG  32 
specifically  stated  otherwise,  and 

when  a  comparison  of  currents  under  any  conditions  is  being 
made  it  is  always  the  saturation  current  that  is  used  unless 
otherwise  definitely  specified.  It  is  extremely  important  to 
observe  this  or  else  serious  confusion  and  error  will  arise. 
The  voltage  necessary  to  produce  saturation  in  each  particular 
instance  should  always  be  tested  as  the  point  of  saturation  will 
be  reached  for  different  voltages  under  different  circumstances. 

56.  Variation  of  Current  with  Distance  Between  the  Plates. 
—Replace  the  plate  P  (Fig.  31)  by  a  sheet  of  wire  gauze  of 
the  same  size  and  turn  P  and  Q  through  a  right  angle  so  that 
the  rays  fall  perpendicularly  upon  Q  after  passing  through 
the  gauze  P  as  shown  in  Fig.  33.  Make  exactly  the  same 
connections  to  the  battery  and  electrometer  as  in  Fig.  31. 
Make  the  potential  between  P  and  Q  equal  to  about  300  volts. 
Place  P  and  Q  about  3  cm.  apart  and  measure  the  saturation 


,      '' 


74 


RONTGEN    RAYS 


FIG.  33. 


current.  Increase  the  distance  to  4  cm.  and  again  measure 
the  saturation  current.  Repeat  this  for  gradually  increasing 
distances  until  the  current  has  been  measured  for  six  or  eight 

different  distances.     Note  that  the  7 

Q.          current  increases  with  the  distance 

between    the    plates,    although    in( 
each  case  it  is  the  saturation  cur*/ 
rent.     If    the    plates    are    not    too 
close  together  to  start  with  the  cur- 
rent in  the  different  cases  will  be 
found  to  be  practically  proportional 
to  the  distance  between  the  plates. 
This  is  quite  different  from  the  cor- 
responding measurement  of  electric 

current  through  a  solid  or  a  liquid.  When  the  distance  between 
two  plates  immersed  in  a  liquid  is  increased  the  current  de- 
creases on  account  of  the  increase  of  resistance  between  the 
plates.  These  experiments  show  however  that  just  the  opposite 
result  takes  place  in  the  case  of  the  current  through  air. 

57.  Theory  of  Ionization. — These  phenomena  along  with 
others  in  connection  with  gases  rendered  conducting  by  the 
action  of  Rontgen  rays  led  J.  J.  Thomson  and  E.  Rutherford 
to  formulate  in  1896  the  ionization  theory  of  gases  which  now 
forms  the  basis  of  the  whole  subject  of  the  conducting  power 
of  gases  and  which  has  become  firmly  established  by  experi- 
ment. According  to  this  theory  the  Rontgen  rays,  when  they 
pass  through  a  gas,  cause  some  of  the  molecules  of  the  gas  to 
be  broken  up  into  positively  and  negatively  charged  carriers  of 
electricity  called  ions.  The  electromagnetic  energy  of  the 
Rontgen  rays  causes  a  negatively  charged  electron  to  be  sepa- 
rated from  the  molecule  of  the  gas  acted  upon,  leaving  the 
remainder  of  the  molecule  positively  charged.  This  process  of 
separation  is  called  ionization,  and  the  gas  is  said  to  be  ionized. 
From  each  molecule  ionized  two  ions  having  equal  charges  but 
of  opposite  sign  are  thus  produced. 

This  theory  explains  very  satisfactorily  the  different  prop- 
erties of  an  ionized  gas.  The  transference  of  electricity 


THEORY   OP   IONIZATION  75 

through  the  gas  is  due  to  the  movement  of  these  charged  car-  J 
riers  under  the  influence  of  an  electric  field.  The  positive  ions 
are  attracted  towards  the  negative  electrode  and  the  negative 
ions  to  the  positive  electrode,  and  the  movement  of  these  elec- 
tric charges  constitutes  a  current.  Since  these  ions  are  charged 
it  is  clear  why  the  conductivity  is  removed  when  the  gas  is 
passed  through  the  tube  with  the  central  wire  between  which 
there  is  an  electric  field,  for  the  positive  and  negative  ions  are 
attracted  to  the  negative  and  positive  electrodes  respectively  — j 
and  thus  removed  from  the  gas.  When  the  gas  containing 
ions  is  passed  through  cotton  wool  or  through  the  water  the — • 
ions  are  caught  and  left  behind.  The  number  of  molecules 
which  become  ionized  even  in  a  strongly  ionized  gas  is  ex- 
tremely small  compared  with  the  total  number  of  molecules 
present,  the  number  being  probably  of  the  order  of  only  two 
or  three  in  a  million.  Thus  when  the  gas  is  passed  through  the 
electric  field  or  wool  filter  there  is  little  chance  for  many  ions 
to  escape  being  caught  and  removed. 

The  gradual  disappearance  of  the  conductivity  of  a  gas  fol- 
lows naturally  from  this  theory.  When  the  ions  are  moving 
about  in  the  gas  if  a  positive  and  a  negative  ion  come  near 
enough  they  will  attract  each  other  and  unite  and  thus  neu- 
tralize each  other.  The  ions  thus  recombine,  and  as  far  as 
their  electrical  effects  are  concerned  disappear. 

This  theory  explains  the  saturation  curve  showing  the  rela- 
tion between  voltage  and  current  through  a  gas  between  two 
plates.  The  greater  the  potential  difference  between  the  plates 
the  greater  the  force  drawing  the  ions  out  of  the  gas  and  con- 
sequently the  faster  they  will  move  towards  the  plates.  The 
faster  they  move  the  greater  will  be  the  number  that  reach  the 
plates  per  second,  and  besides  by  moving  more  quickly  the  less 
will  be  the  chance  of  their  recombining  and  therefore  there 
will  be  more  to  reach  the  plates  per  second.  The  number  of 
ions  that  reach  the  plates  per  second  will  therefore  be  pro- 
portional to  the  potential.  The  current  is  proportional  to  the 
charge  given  to  the  plates  per  second  and  therefore  to  the 
number  of  ions  arriving  at  the  plates  each  second.  The  cur- 


76  RONTGEN  RAYS 

rent  is  therefore  proportional  to  the  potential  at  the  lower 
voltages.  When  however  the  voltage  reaches  a  certain  amount 
the  ions  move  so  fast  that  they  practically  all  reach  the  plates 
before  they  have  time  to  recombine.  At  such  a  voltage  all 
the  ions  will  be  removed  and  the  current  therefore  can  not 
be  increased  further  by  increase  of  voltage.  The  current 
will  therefore  reach  a  maximum  and  remain  so  for  any 
increase  in  the  potential. 

The  explanation  of  the  increase  of  current  between  two 
plates  when  the  distance  between  them  is  increased  is  obvious 
from  this  theory.  When  the  distance  is  increased  the  volume 
of  gas  acted  upon  by  the  rays  is  increased  and  consequently 
the  number  of  ions  produced  grows  larger  in  the  same  pro- 
portion. There  are  more  ions  between  the  plates  and  there- 
fore a  greater  number  will  be  drawn  to  the  plates  in  unit  time. 

58.  Absolute  Measure  of  Current. — Instead  of  simply  com- 
paring the  different  currents  through  an  ionized  gas  their 
value  may  easily  be  determined  in  absolute  measure.  If  the 
gas  is  very  strongly  ionized  the  current  may  in  a  few  cases  be 
large  enough  to  be  measured  by  a  very  sensitive  galvanometer. 
Under  these  circumstances  replace  the  electrometer  and  con- 
denser in  Fig.  31  by  the  sensitive  galvanometer,  and,  know- 
ing the  constants  and  calibration  of  the  galvanometer,  deter- 
mine the  saturation  current  in  the  same  manner  as  you  would 
measure  the  current  in  a  metallic  conductor.  As  a  rule  how- 
ever the  ionization  current  is  too  small  to  be  measured  by  this 
method  and  the  electrometer  method  must  be  used.  It  has 
been  shown  (§20)  that  if  d  be  the  number  of  scale  divisions 
passed  over  per  second  by  the  electrometer  needle  as  the  insu- 
lated quadrants  charge  up  to  any  given  potential,  and  d±  the 
number  of  divisions  corresponding  to  a  potential  of  one  volt 
on  these  quadrants,  and  C  the  total  capacity  in  microfarads  of 
the  system  then  the  current  i=  C/io6  X  d/d1  amperes.  Using 
the  apparatus  shown  in  Fig.  31  make  a  series  of  measurements 
of  different  currents  in  amperes  as  follows :  Run  the  X  ray 
bulb  a  few  times  for  fifteen-second  intervals  with  a  rest  of 
fifteen  or  twenty  seconds  between  so  as  to  get  it  running  uni- 


USE   OF   GUARD-RING  77 

formly.  Then  start  to  take  readings.  Earth  the  plate  Q  and 
electrometer  by  the  key  K  and  see  that  the  electrometer  needle 
is  at  rest.  Start  the  bulb  and  let  it  run  for  five  seconds.  At 
the  end  of  the  five  seconds,  without  stopping  the  bulb,  pull  the 
cord  which  opens  the  key  K,  thus  insulating  the  plate  Q  and 
allowing  it  to  charge  up.  Let  it  charge  up  for,  say,  ten  sec- 
onds, and  at  the  end  of  this  time  shut  off  the  bulb  by  opening 
the  switch  which  controls  the  coil.  When  the  needle  comes  to 
rest  take  the  reading.  Then  earth  the  electrometer  again  to 
bring  the  needle  to  zero.  Take  the  time  carefully  in  these 
observations  by  means  of  a  stop  watch  or  accurate  chronom- 
eter. The  number  of  divisions  d  are  thus  obtained.  Repeat 
this  procedure  in  each  case.  Determine  d±  by  means  of  the 
standard  cell  as  described  in  §  17.  Measure  the  capacity  C  of 
the  system  including  the  plate  Q,  the  condenser  C  and  the  elec- 
trometer by  one  of  the  methods  described  in  §  19.  Supply 
these  values  obtained  in  the  above  equation  and  determine  t  in 
amperes.  The  following  may  be  given  as  a  typical  example  of 
such  a  determination.  Suppose  the  number  of  scale  divi- 
sions moved  over  in  ten  seconds  is  150,  which  would  be  a  con- 
venient rate;  therefore  ^=15.  Let  ^  =  600  scale  divisions 
and  let  C— .001.  Then  *  will  be  .ooi/io6  X  15/600  which 
equals  2.5  X  IO"11  amperes.  Currents  considerably  less  than 
this  may  be  measured  with  accuracy  as  a  much  slower  rate 
than  fifteen  divisions  per  second  may  be  observed  with  great 
ease. 

59.  Guard-ring  Method. — In  measuring  the  ionization  cur- 
rent between  two  plates  a  still  further  and  important  precau- 
tion must  be  taken  in  the  arrangement  of  the  plates  in  order 
to  secure  greater  accuracy.  When  a  difference  of  potential 
is  established  between  two  parallel  plates  the  electric  field  is 
not  strictly  perpendicular  to  the  plane  of  the  plates  over  the 
whole  area  of  the  plates,  but  only  over  the  central  portion, 
the  lines  of  force,  near  the  edges  spreading  out  in  curved  lines. 
The  above  measurements  are  made  on  the  assumption  that 
the  field  is  uniform,  therefore  only  the  portion  of  the  ionized 
gas  in  the  space  between  the  central  areas  should  be  used. 


78  RONTGEN   RAYS 

This  may  be   done  by  using  what  is   called  the  guard-ring 
method.     The  central  portion  of  the  plate  Q  is  cut  out,  leav- 
ing a  margin  3  or  4  cm.  in  width.    The  part  cut  out  is  replaced 
by  a  plate  A,  Fig.  34,  which  is  just  a 
little  smaller  than  the  opening,  so  that 
there  is  a  space  all  round  of  only  about 
0.5  mm.     This  central  portion  should  be 
carefully  insulated  which  may  be  done 
conveniently    by    supporting    it    by    U- 
shaped  pieces  of  ebonite  fastened  to  it- 
self  and  to   the   outer   rim  by   screws. 
This  central  part  A  is  then  connected  to 
FlG-  34-  the  electrometer,  while  the  outer  rim  is 

connected  to  earth.  The  plate  P  re- 
mains the  same  size  as  the  outer  rim.  The  ionization  is  thus 
measured  only  throughout  the  central  part  of  the  field  over 
the  area  of  A  where  the  field  is  uniform.  This  principle  of 
the  guard-ring  is  applicable  in  a  great  variety  of  cases,  and 
in  all  such  cases  as  we  have  been  considering  where  the  uni- 
formity of  the  field,  is  necessary  it  should  be  used. 

60.  Dependence  of  Ionization  on  Quality  of  the  Rays. — The 
number  of  ions  produced  in  a  given  volume  of  air,  and  there- 
fore the  current  through  it,  depends  to  a  great  extent  upon  the 
quality  of  the  rays  employed.  "  Soft  "  rays  are  more  efficient 
ionizers  than  "  hard  "  or  penetrating  rays.  To  secure  a  large 
amount  of  ionization  in  a  gas  it  is  best  to  use  a  moderately 
"soft"  bulb.  If  several  bulbs  are  available  which  give  out 
rays  of  different  qualities  measure  the  ionization  produced  by 
each  one  in  turn  in  the  same  space  between  the  guard-ring 
plates  and  under  the  same  conditions.  If  the  automatic  bulb  of 
the  type  shown  in  Fig.  30  is  available  vary  the  quality  of  the 
rays  by  varying  the  length  of  the  spark  gap  between  the  main 
cathode  and  the  auxiliary  cathode.  With  a  short  spark  length 
the  rays  given  out  are  softer  than  for  a  longer  spark  gap,  as 
with  a  short  spark  length  the  pressure  in  the  bulb  is  compara- 
tively higher.  Measure  and  compare  the  amount  of  ioniza- 
tion produced  when  different  lengths  of  spark  gap  are  used, 


ABSORPTION    OF   RONTGEN    RAYS  79 

that  is  for  different  qualities  of  rays.  Unless  the  rays  are  so 
soft  that  they  are  diminished  too  much  in  intensity  by  absorp- 
tion the  softer  rays  should  be  found  to  produce  more  ioniza- 
tion  than  the  very  hard  rays. 

In  each  case  immediately  after  measuring  the  ionization  and 
before  altering  the  spark  gap  place  a  sheet  of  brass  or  alumin- 
ium in  the  path  of  the  rays  between  the  bulb  and  the  guard- 
ring  plates  and  measure  the  ionization  produced  by  the  rays 
after  passing  through  the  plates.  If  the  rays  are  very  pene- 
trating a  great  proportion  of  them  will  go  through  the  plate 
and  be  effective  as  ionizers  on  the  other  side,  but  if  they  are 
not  very  penetrating  a  very  small  proportion  will  get  through 
and  therefore  the  ionizing  effect  will  be  cut  down.  The 
ratio  in  which  the  ionization  is  decreased  by  passing  the  rays 
through  the  metal  sheet  will  be  a  measure  in  the  inverse  ratio 
of  the  penetrating  power  of  the  rays.  By  means  of  these 
measurements  compare  the  relative  penetrating  power  of  the 
rays  with  their  ionization  power  for  rays  of  different  qualities. 

61.  Absorption  of  Rontgen  Rays,  (a)  By  Solids. — Different 
materials  absorb  Rontgen  rays  of  any  given  type  by  different 
amounts.  An  approximate  qualitative  comparison  of  the  rela- 
tive absorbing  powers  of  bodies  has  been  given  in  §  43.  We 
are  now  in  a  position  to  measure  these  absorptive  powers  more 
definitely.  Place  the  usual  set  of  guard-ring  plates  about  18  or 
20  cm.  in  front  of  the  window  in  the  usual  position.  Measure 
the  saturation  current  between  the  plates,  then  place  a  sheet  of 
the  absorbing  material  to  be  tested  of  a  given  thickness  in  the 
path  of  the  rays  between  the  bulb  and  the  plates  and  measure 
the  saturation  current  again.  A  comparison  of  the  currents  in 
the  two  cases  will  give  the  percentage  of  the  rays  absorbed,  as 
the  decrease  in  the  ionization  will  be  proportional  to  the  de- 
crease in  the  quantity  of  rays  due  to  absorption.  Secure  as 
large  a  variety  as  possible  of  sheets  of  different  kinds  of  metal, 
glass,  wood,  etc.,  all  of  the  same  thickness  of  about  2  or  3  mm. 
Compare  the  absorption  produced  by  the  different  materials 
and  note  carefully  the  great  range  of  absorbing  powers.  Re- 
peat this  series,  using  a  different  quality  of  rays  and  note  that 


So  RONTGEN    RAYS 

the  relative  penetrating  powers  may  not  necessarily  be  the 
same  for  different  kinds  of  rays.  Make  a  comparative  table 
of  these  measurements  for  reference. 

Now  select  any  one  material  which  may  be  obtained  in  thin 
sheets  of  uniform  thickness,  such  as  tinfoil  or  aluminium  foil. 
Measure  the  absorption  produced  by  a  single  sheet  of  tinfoil, 
then  introduce  another  sheet  and  measure  the  absorption  pro- 
duced by  the  double  thickness.  Continue  thus  adding  a  sheet 
at  a  time  and  observe  how  the  absorption  increases  with  the 
thickness.  It  will  be  found  that  this  absorption  does  not  in- 
crease in  direct  proportion  to  the  thickness.  If  a  bulb  of  me- 
dium hardness  is  used  it  will  be  found  that  the  first  few  layers 
of  tinfoil  produce  a  greater  percentage  effect  than  the  remain- 
ing layers,  owing  to  the  fact  that  there  is  a  mixture  of  rays  of 
different  penetrating  powers  and  the  softer  rays  are  largely 
absorbed  by  a  few  thicknesses  of  metal,  while  the  harder  ones 
pass  through  with  little  absorption,  and  after  the  softer  rays 
are  cut  out  more  layers  in  proportion  have  to  be  introduced 
to  cut  down  the  intensity  of  the  more  penetrating  rays  to  the 
same  amount.  Measure  the  absorption  produced  by  the  dif- 
ferent substances  in  this  way.  Repeat  these  measurements, 
using  different  qualities  of  rays. 

(b)  Standard  Test. — In  making  a  series  of  measurements 
such  as  the  above,  in  order  to  obtain  any  reliable  comparisons, 
the  source  of  the  rays  must  remain  constant  throughout,  or 
if  it  does  not  there  must  be  some  means  of  checking  any 
change  in  the  source  and  correcting  for  it.  For  this  reason 
a  separate  test  apparatus  should  be  introduced  to  detect  and 
correct  for  any  change  if  it  does  occur.  A  convenient  arrange- 
ment for  this  purpose  is  shown  in  Fig.  35.  P  and  Q  are  the 
guard-ring  plates  corresponding  to  the  plates  P  and  Q  in  Fig. 
31,  the  connections  of  which  are  the  same  as  before.  Another 
set  of  plates  R  and  5  exactly  similar  to  P  and  Q,  but  only 
about  one  half  or  one  third  the  size,  are  introduced  in  the  posi- 
tion shown  and  form  a  standard  test  apparatus.  5*  is  connected 
to  a  separate  condenser  and  to  a  key  //of  the  form  shown  in 
Fig.  ii  (b),  so  that  £  may  be  insulated  from  or  connected  to 


UNIVERSITY 

OF 


STANDARD   TEST   APPARATUS 


Si 


the  electrometer  at  will.  If  H  is  raised  then  S  is  insulated,  and 
while  the  rays  are  acting  it  will  charge  up.  If  H  is  closed 
while  K  is  open  this  charge  is  thrown  into  the  electrometer  and 
it  may  be  measured.  The  test  of  the  constancy  of  the  rays 
should  be  made  in  the  following  manner :  Run  the  bulb  in  the 
usual  way  and  open  the  keys  H  and  K  at  the  same  instant.  S 
charges  up  for  the  same  time  as  Q  but  is  insulated  from  Q. 
Take  the  reading  for  Q  and  then  discharge  Q  by  earthing  it 


ft 

\      -1_ 

y 

i    j 

1 

|    .T.      ^ 

I 

,   -     4k 

S 

5.  

T  ^P 

i 

R 

p 

J 

f  ARTH 

HHhj 

V 

FIG.  35. 

through  K.  When  the  needle  comes  to  rest  insulate  Q  again 
and  then  close  H,  thus  throwing  the  charge  contained  by  5" 
and  its  condenser  into  the  electrometer.  Take  the  reading  for 
this  charge.  This  gives  a  measure  of  the  ionizing  power  of 
the  rays  between  S  and  R.  For  each  measurement  of  current 
between  P  and  Q  make  the  simultaneous  measurement  between 
R  and  S.  If  any  change  in  the  rays  occurs  it  will  be  detected 
by  the  standard  RS,  and  a  correction  may  be  made  for  the 
readings  for  the  current  between  P  and  Q.  The  absorbing 
body  which  is  to  be  tested  is  placed  in  the  space  /  between  P 
and  R.  The  introduction  of  these  absorbing  bodies  thus  does 
not  affect  the  conditions  between  R  and  S,  while  it  does  be- 
tween P  and  Q.  If  the  test  apparatus  were  absent  one  could 
not  be  certain  whether  any  change  in  the  ionization  between 
P  and  Q  were  due  entirely  to  the  absorption  of  the  body  at 
/  or  partly  due  to  this  and  partly  due  to  a  change  in  the  source 
of  the  rays.  In  all  measurements  of  this  type  this  test  ap- 
paratus should  be  used  as  a  check  on  the  rays. 
7 


82  RONTGEN    RAYS 

(c)  By  Liquids. — Liquids  also  absorb  Rontgen  rays  to  a 
considerable  extent.    The  absorption  produced  by  liquids  may 
be  measured  in  exactly  the  same  way  as  that  produced  by 
solids,  by  placing  the  liquid  contained  in  a  cell  with  parallel 
glass  sides  in  the  space  /  and  testing  as  before.   In  this  instance 
to  obtain  the  absorption  produced  by  the  liquid  alone  the  ab- 
sorption produced  by  the  cell  when  empty  must  be  separately 
measured  and  subtracted  from  the  total  absorption  produced 
by  the  cell  plus  the  contained  liquid.    Compare  in  this  way  the 
absorption  produced  by  various  liquids.    Also  select  any  given 
solution  and  measure  the  absorption  for  different  concentra- 
tions of  the  solution. 

(d)  By    Gases. — Rontgen   rays   are   absorbed   by   gases   in 
their  passage  through  them,  but  of  course  to  a  very  much  less 
extent  than  by  solids  or  liquids.     Suppose  that  /  is  the  energy, 
or  intensity,  of  the  rays,  and  when  they  pass  through  unit 
length  of  the  gas  a  fraction  A/  of  the  energy  is  absorbed,  then 
a  small  change  dl  in  the  intensity  in  passing  through  a  small 
distance  dx  is  given  by  the  relation  dl  =  —  \ldx,  since  dl  is 
a  decrease  while  dx  is  an  increase. 

dl 
Therefore  -j-  =  —  \dx; 

Therefore  by  integration  log  7  +  c  =  —  A.r  where  c  is  the  con- 
stant of  integration.  Therefore  if  70  is  the  intensity  of  the 
rays  when  x  =  o,  that  is  the  intensity  before  any  absorption 
takes  place,  • 

log  /  —  log  70  =  — A,r. 

Therefore  -    =  e-A* 


and  /=  70e-A*. 

The  fraction  A  is  usually  termed  the  coefficient  of  absorption. 
It  is  a  comparatively  small  number,  and  it  is  not  so  easy  to 
measure  by  the  same  method  as  was  used  in  the  case  of  solids 


ABSORPTION    BY    GASES 


and  liquids  as  gases  produce  such  a  comparatively  small 
amount  of  absorption.  The  following  experimental  method 
used  by  Rutherford  and  the  author  is  a  convenient  one  for 
determining  A  and  comparing  the  absorptive  powers  of  gases 
under  different  conditions.  The  arrangement  of  the  apparatus 
is  shown  in  Fig.  36.  A  and  A'  are  two  brass  tubes  about  5  or 
6  cm.  in  diameter  and  about  one  meter  long.  The  ends  are 
closed  by  aluminium  caps  about  I  mm.  thick.  These  tubes 
should  be  made  as  nearly  as  possible  exactly  alike  and  should 
be  gas-tight.  They  are  placed  symmetrically  with  regard  to 
the  bulb,  so  the  central  axis  of  each  passes  through  the  center 
of  the  anode  of  the  bulb.  CD  and  C'D'  are  two  exactly 
similar  sets  of  guard-ring  plates  for  determining  current.  C 
and  D'  are  connected  to  opposite  poles  of  the  battery.  D  and 
C'  are  connected  together  and  to  the  electrometer.  The  guard- 


N 


L      M 


FIG.  36. 

rings  are,  as  usual,  connected  to  earth.  When  the  air  between 
these  two  sets  of  plates  is  ionized  the  plates  D  and  C'  will 
receive  charges  of  opposite  sign,  and  if  the  amount  of  ioniza- 
tion  is  the  same  in  both  cases,  the  charges  will  be  equal  and  the 
electrometer  needle  will  show  no  deflection.  M  and  N  are 
thick  lead  screens  to  prevent  any  stray  rays  from  reaching  the 
test  plates. 

Place  the  several  parts  of  the  apparatus  in  position  as  sym- 


84  RONTGEN   RAYS 

metrically  as  possible;  then  test  whether  the  electrometer 
needle  balances  under  the  influence  of  the  two  opposite  ioniza- 
tion  currents.  If  not,  adjust  the  position  of  the  two  sets  of 
plates  by  trial  until  a  balance  is  obtained.  The  rays  in  passing 
through  the  tube  are  absorbed  by  the  gas  in  the  tube.  If  the  air 
be  exhausted  from  A  the  absorption  in  A  will  be  lessened  and 
the  intensity  of  the  rays  which  reach  CD  will  be  greater  than 
the  intensity  of  those  which  reach  C'D',  and  the  balance  will  be 
destroyed.  If  both  tubes  are  completely  exhausted  the  bal- 
ance should  be  restored. 

Suppose  that  /  is  the  intensity  of  the  rays  which  reach  either 
set  of  plates  when  both  tubes  are  completely  exhausted  ;  then 
when  both  tubes  are  full  of  air  at  atmospheric  pressure  the 
intensity  will  be  /c~Xd,  where  A  is  the  coefficient  of  absorption 
and  d  the  length  of  air  passed  through.  Since  the  currents 
between  C  and  D  and  between  Cr  and  D'  are  proportional  to 
the  intensity  of  radiation,  therefore  if  A  is  exhausted  and  A' 
full  of  air  the  difference  between  the  currents  will  be  pro- 
portional to  the  difference  between  the  intensities  /  and  7e~^, 
that  is,  to  /  —  /€~x<*.  Suppose  again  that  the  rays  coming 
through  A'  be  completely  cut  off  by  a  lead  screen  while  A  is 
still  completely  exhausted,  then  the  current  between  C  and  D 
will  be  proportional  to  the  intensity  /  when  no  absorption  takes 
place,  and  therefore 

Diff.  bet.  currents  when  A  is  exhausted  and  A'  full  of  air 
Total  current  when  A  is  exhausted  and  rays  through  A  cut  off 


Let  d±  =  rate  of  movement  of  the  needle  when  A  is  ex- 

hausted and  A'  full  of  air, 
Let  c?2  =  rate  of  movement  when  the  rays  through  A'  are 

cut  off  by  lead  screen. 


Then 


Therefore 


IONIZAT1ON    AND   PRESSURE  85 

l          ' 


then  by  expansion  \d  =  -^  , 

since  A  is  very  small  and  terms  beyond  the  first  power  may  be 
neglected. 

Start  with  both  tubes  full  of  air  and  adjust  till  a  balance 
is  obtained.  Exhaust  A  and  observe  the  deflection  dt.  Then 
place  a  thick  lead  screen  between  A'  and  the  plates  CD'  and 
observe  the  deflection  dz.  Measure  the  length  d  of  the  tube 
and  calculate  A. 

No  constant  standard  value  can  be  given  for  this  coefficient, 
for  it  varies  with  the  quality  of  the  rays.  The  softer  the  rays 
the  larger  will  be  the  value  of  A.  Rutherford  and  the  author, 
working  with  fairly  penetrating  rays,  obtained  a  value  equal 
to  0.000279,  but  the  coefficient  may  be  much  larger  with 
softer  rays. 

Measure  the  coefficients  of  absorption  for  any  other  gases 
which  may  be  conveniently  obtained.  Compare  also  the  ab- 
sorbing powers  of  different  gases  by  first  filling  both  tubes 
with  air  at  atmospheric  pressure  and  balancing  and  then  re- 
placing the  air  in  A  by  other  gases  in  turn  and  observing  the 
alteration  in  balance  in  each  case. 

62.  Dependence  of  lonization  on  Pressure  of  the  Gas.  —  For 
a  given  type  of  rays  the  amount  of  ionization  depends  upon 
the  pressure  of  the  gas.  The  number  of  molecules  in  a  given 
volume  is  proportional  to  the  pressure  of  the  gas  and  conse- 
quently the  number  of  molecules  to  be  ionized  increases  with 
the  pressure  and  an  increase  in  ionization  with  increase  of 
pressure  is  to  be  expected.  To  measure  this  experimentally  an 
air-tight  ionization  chamber  will  be  necessary.  A  suitable  one 
is  shown  in  Fig.  37.  AB  is  a  brass  cylinder  about  15  cm. 
diameter  and  30  cm.  long,  the  walls  being  about  2  mm.  thick. 
The  end  B  is  closed  by  a  brass  plate  soldered  or  brazed  to  the 
cylinder.  At  the  other  end  a  heavy  brass  flange  cd  about  0.5 


86 


RONTGEN   RAYS 


cm.  thick  and  3  cm.  in  width,  fitting  the  cylinder  tightly,  is 
soldered  to  it.  An  aluminium  plate  about  0.5  cm.  thick  and  of 
the  same  diameter  as  the  flange  is  made  to  fit  closely  on  the 
face  of  the  flange,  so  it  may  be  bolted  firmly  to  it.  These  sur- 
faces should  be  turned  as  smooth  as  possible  to  obtain  a  close 
fit.  The  central  part  of  this  plate  is  recessed  down  to  a  thick- 


1 

f      1 

t 

J 

-"-" 

? 

-  / 

x-------"1 

>-'. 

1 

i 

« 

ir/ 
•  , 

.  TO  PUfA  ? 


FIG.  37. 

ness  of  about  I  mm.  over  an  area  of  6  or  8  cm.  in  diameter  so 
as  to  allow  the  rays  to  pass  into  the  cylinder  without  being 
diminished  too  much  in  intensity  in  passing  through  the  alu- 
minium. The  joint  between  the  plate  and  the  flange  may  be 
made  air-tight  by  placing  a  soft  lead  wire  about  I  mm.  in  diam- 
eter in  a  circle  on  the  flange  inside  the  bolts  and  then  placing 
the  aluminium  plate  on  the  wire  and  bolting  it  down  tightly 
until  the  wire  is  squeezed  down  to  about  half  its  original  thick- 
ness, as  described  in  §  31.  T  and  T'  are  two  side  tubes  5  cm. 
long  and  placed  diametrically  opposite.  P  and  P'  are  a  set  of 
guard-ring  plates  about  6  cm.  apart  made  of  aluminium.  5  and 
S'  are  two  stout  brass  rods  which  pass  through  ebonite  plugs  in 
the  ends  of  the  tubes  T  and  T'  and  support  the  plates  P  and  P'. 
These  rods  should  fit  tightly  in  the  ebonite  plugs  and  may  be 
made  to  screw  into  the  face  of  the  plates  P  and  P'.  The  ebon- 
ite plugs  and  the  inside  of  the  tubes  T  and  T'  should  be 
threaded  so  that  the  plugs  may  screw  into  the  tubes  to  with- 
stand the  strain  when  the  gas  pressure  is  increased  from  within. 
If  all  these  joints  are  carefully  made  to  fit  tightly  they  can 


IONIZATION   AND   PRESSURE  87 

finally  be  made  perfectly  air-tight  by  waxing  with  paraffin. 
The  plates  P  and  P'  are  connected  in  the  usual  way  through  5* 
and  Sr  to  the  electrometer  and  battery  respectively. 

Place  this  apparatus  a  short  distance  in  front  of  the  Rontgen 
ray  bulb  and  very  carefully  adjust  the  position  of  the  cylinder 
and  the  size  of  the  cone  of  rays  so  that  the  rays  will  pass 
between  the  plates  without  touching  them.  This  adjustment 
is  very  important,  in  order  to  avoid  any  secondary  radiation  at 
the  surface  of  the  plates,  and  it  may  be  very  accurately  made 
by  calculating  the  distances  and  dimensions  for  the  cone  of 
rays  and  leaving  a  sufficient  margin  for  safety.  Place  the 
standard  test  apparatus  between  the  bulb  and  the  cylinder  so 
as  to  test  the  constancy  of  the  rays  during  the  experiment. 

Starting  with  the  air  at  atmospheric  pressure  measure  the 
saturation  current  between  the  plates.  Then  reduce  the  pres- 
sure by  about  10  cm.  by  the  air  pump  and  measure  the  satura- 
tion current  and  also  measure  the  pressure  of  the  air.  Repeat 
this  until  the  pressure  is  reduced  to  a  centimeter  or  less.  Also 
increase  the  pressure  by  stages  above  atmospheric  pressure  by 
pumping  air  into  the  cylinder,  and  measure  the  saturation  cur- 
rent at  the  different  pressures  until  a  pressure  of  two  or  three 
atmospheres  is  reached.  At  each  measurement  test  the  inten- 
sity of  the  rays  by  the  standard  test  apparatus  and  if  any 
change  occurs  correct  for  it.  More  than  one  reading  should 
be  taken  at  each  pressure  and  the  mean  taken  to  ensure  greater 
accuracy.  Plot  a  curve  showing  the  relation  between  ioniza- 
tion  and  pressure.  This  curve  should  be  a  straight  line  show- 
ing that  the  ionization  is  proportional  to  the  pressure. 

63.  Dependence  of  Ionization  on  the  Nature  of  the  Gas. — 
Another  important  factor  on  which  the  ionization  depends  is 
the  nature  of  the  gas  ionized.  For  any  given  type  of  rays  the 
amount  of  ionization  produced  in  a  given  volume  of  gas  under 
constant  conditions  is  quite  different  for  the  different  gases. 
The  heavier  gases,  as  we  have  seen,  absorb  the  rays  more  than 
do  the  lighter  gases,  and  the  greater  the  absorption  the  greater 
the  ionization.  The  ionization  produced  in  different  gases  may 
be  measured  by  means  of  the  apparatus  used  in  the  last  experi- 


88  RONTGEN   RAYS 

ment  (Fig.  37).  Starting  with  air  at  atmospheric  pressure 
measure  the  saturation  current.  Then  exhaust  the  cylinder 
as  rapidly  as  possible  and  refill  with  another  gas,  say  hydrogen, 
and  measure  the  current.  Repeat  this  for  all  the  different 
gases  which  may  be  conveniently  used,  avoiding,  of  course, 
any  gases  which  will  act  upon  the  metals  in  the  vessel.  The 
relative  ionization  in  the  different  gases  will  thus  be  obtained 
by  reducing  these  readings  to  the  same  scale. 

In  replacing  one  gas  by  another  it  is  best  to  exhaust  and 
refill  two  or  three  times,  so  as  to  ensure  that  no  trace  of  the 
preceding  gas  remains.  Make  sure  also  that  .the  pressure  is 
the  same  in  all  cases.  Keep  the  temperature  as  nearly  con- 
stant as  possible.  In  filling  the  cylinder  with  gas  dry  the  gas 
very  thoroughly  by  passing  it  through  a  series  of  drying 
agents  as  it  passes  into  the  cylinder.  If  this  precaution  is 
neglected  serious  trouble  will  result  due  to  the  destruction  of 
the  insulation  inside  the  cylinder,  as  well  as  the  complication 
of  results  by  the  different  amounts  of  moisture  present  at  dif- 
ferent times.  As  usual  test  the  rays  at  each  reading  by  the 
standard  test  apparatus. 

Use  a  different  quality  of  Rontgen  rays  either  by  using 
another  bulb  giving  out  a  different  type  of  rays  or  by  altering 
the  spark  gap  in  the  self-regulating  bulb  and  repeat  the  experi- 
ments with  the  same  set  of  gases.  If  the  rays  used  in  the  two 
sets  of  readings  differ  considerably  in  quality  it  will  be  found 
that  the  set  of  numbers  showing  the  relative  ionization  in  the 
second  case  will  differ  somewhat  from  those  obtained  in  the 
first  instance. 

These  experiments  show  that  in  general  the  denser  the  gas 
the  greater  the  amount  of  ionization  produced  by  Rontgen 
rays.  The  relative  ionization  is  not  however  proportional  to 
the  relative  densities  of  the  gases.  The  relative  ionization  de- 
pends upon  the  quality  of  the  rays  used.  The  more  pene- 
trating the  rays  the  more  nearly  does  the  ionization  become 
proportional  to  the  density  of  the  gases,  but  no  rays  have  as 
yet  been  found  to  give  exact  proportionality. 

64.  Recombination  of  Ions,     (a)   Theory. — We  have  seen 


RECOMBINATION    OF   IONS  89 

(§47)  that  the  conductivity  imparted  to  a  gas  by  Rontgen  rays 
persists  for  a  short  time  after  the  rays  have  ceased.  The  con- 
ductivity gradually  disappears,  and  the  greater  the  quantity  of 
ionization  the  more  rapid  will  be  the  rate  at  which  it  dies 
away.  If  a  comparatively  weak  source  of  rays  be  used  it  is 
found  that  when  the  rays  begin  to  ionize  the  gas  the  saturation 
current  gradually  increases,  showing  that  the  ions  gradually 
increase  in  number  until  a  steady  state  is  reached,  when  no 
further  increase  in  the  number  will  take  place,  no  matter  how 
long  the  rays  act.  As  the  rays  are  continually  producing  ions 
they  must  be  disappearing  at  the  same  rate  as  they  are  being 
produced  when  the  steady  condition  is  reached.  What  becomes 
of  them?  They  are  positively  and  negatively  charged  bodies 
and  in  moving  about  in  the  gas  must  collide  with  one  another. 
When  a  positive  and  negative  ion  come  together  they  neutral- 
ize each  other  electrically  and  disappear,  as  far  as  producing 
any  conductivity  is  concerned.  The  rate  at  which  this  recom- 
bination of  the  ions  takes  place  is  an  important  factor  in  deter- 
mining the  current  through  a  gas.  When  the  number  of  ions 
is  large  the  chances  of  collision  are  increased  and  the  rate  of 
recombination  is  consequently  greater  than  when  the  number 
of  ions  is  small. 

The  positive  and  negative  ions  per  cubic  centimeter  are  equal 
in  number.  Suppose  the  number  of  each  is  n.  Then  the 
chances  of  collision  taking  place  will  be  proportional  to  n  x  n. 
Therefore  the  rate  at  which  they  recombine  will  be  propor- 
tional to  n2  and  will  consequently  equal  an2,  where  a  is  a  con- 
stant independent  of  n.  This  constant  a  is  called  the  coefficient 
of  recombination  and  is  the  fraction  of  collisions  which  result 
in  recombination.  Suppose  that  q  is  the  number  of  either 
positive  or  negative  ions  produced  per  second  per  c.c.  at  any 
stage  of  the  ionization.  Then  the  rate  of  change  in  the  number 
of  ions  must  equal  q  —  an2.  But  the  rate  at  which  the  ions 
change  is  equal  to  dn/dt,  where  dn  is  the  small  change  in  num- 
ber in  the  time  dt.  Therefore, 


(0 


90  RONTGEN   RAYS 

If  the  rays  be  suddenly  cut  off  then  q  becomes  zero  and 

dn 


dn 
therefore  —%  =  —  adt. 

Therefore  by  integration  —  i/n  =  —  at  -f-  c  where  c  is  a  con- 
stant and  n  is  the  number  of  ions  per  c.c.  at  a  time  t  after  the 
rays  cease.  Let  N  =  the  maximum  number  of  ions  per  c.c.  at 
the  instant  the  rays  cease,  that  is  when  t  =  o. 

Therefore  I  _  1  —  a/.  (2) 

;/      N 

This  equation  gives  the  number  of  ions  n  per  c.c.  at  any  time 
t  after  the  ionizing  source  has  ceased  to  act  if  a  and  TV  are 
known. 

Again  suppose  that  before  the  rays  cease  a  steady  state  exists, 
that  is,  the  rate  of  production  is  equal  to  the  rate  of  recom- 
bination. Under  these  conditions  no  change  is  taking  place 
in  the  number  of  ions  present  per  c.c.,  and  consequently 
dn/dt  =  o,  and  therefore,  by  equation  (i),  q  =  aN2,  since  in 
the  steady  state  n  =  the  maximum  number  N. 


Therefore  *~J/*'  (3) 

If  the  ions  are  produced  within  a  given  volume  of  gas  between 
two  electrodes  such  as  two  parallel  plates,  and  since  q  is  the 
number  produced  per  second  per  c.c.,  then  the  quantity  of  elec- 
tricity produced  per  c.c.  per  second  will  equal  q  X  e  where  e  is 
the  charge  on  each  ion.  Therefore  if  the  saturation  current  c 
between  the  plates  be  measured  in  absolute  measure  q  x  £  will 
be  proportional  to  this  current  and  therefore  q  X  e  —-  kc  where 
k  is  a  constant. 

Again  if  the  ionization  be  allowed  to  reach  the  steady  state 


RECOMBINATION   OF   IONS  91 

with  no  difference  of  potential  acting  between  the  plates,  then 
N  will  be  the  number  of  ions  actually  existing  per  c.c.  at  any 
instant  during  that  steady  state,  and  therefore  N  X  e  will  be 
proportional  to  the  total  quantity  of  electricity  existing  in  the 
given  volume  of  gas.  If  the  rays  be  suddenly  stopped  and  at 
exactly  the  same  instant  a  high  potential  be  established  between 
the  plates  so  that  all  the  ions  are  suddenly  driven  to  the  plates 
before  any  recombination  can  take  place  then  N  X  e  will  be 
proportional  to  the  whole  charge  clt  measured  in  absolute  units, 
received  by  the  plate.  Therefore  N  X  e  =  k1c1,  where  ^  is  a 
constant.  Therefore  supplying  these  values  for  q  and  N  in 
equation  (3)  a  will  equal 

kc 


toy 


k         C 

Therefore         a  =  TT  •  —  2  •  e  ', 


c  k 

=  K-—-e     where  K—. 


This  constant  K  depends  upon  the  dimensions  of  the  ap- 
paratus and  its  position  relative  to  the  Rontgen  ray  bulb.  By 
measuring  c  and  c±  and  determining  K  for  the  particular  appa- 
ratus the  value  of  a  may  be  determined  in  terms  of  e,  the 
charge  on  an  ion. 

(b)  Experimental  Determination.  —  The  actual  experimental 
determination  of  a  is  somewhat  troublesome,  presenting  sev- 
eral difficulties,  and  extreme  care  must  be  taken  to  secure  satis- 
factory results.  It  has  been  measured  in  absolute  measure 
independently  by  Townsend,  Langevin  and  the  author,  each 
one  using  a  different  method,  the  author  using  the  one  de- 
scribed above.  The  values  obtained  by  these  for  a  in  air  at 
ordinary  pressure  and  temperature  were  respectively  3420  X  e, 
3200  x  e  and  3384  x  e,  where  e  is  charge  carried  by  an  ion. 


RONTGEN    RAYS 


The  following  experiment  will  show  in  a  fairly  simple 
manner  the  rate  at  which  the  ions  recombine  and  may  be  used 
to  verify  the  formula  in  equation  (2).  AB  (Fig.  38)  is  a 
brass  tube  about  4  cm.  in  diameter  and  about  a  meter  long. 


A 

D  i 

VJLX 

ES 

!  !  I     E 
«  *  i 

Jj-^f  

"a                   ~b                        C.                       d 

EARTH 


B 


EARTH 


FIG.  38. 


Small  brass  rods  a,  b,  c  and  d,  all  exactly  alike  and  about  6  or 
8  cm.  long,  are  held  in  place  on  the  axis  of  the  tube  by  brass 
rods  passing  through  ebonite  plugs  in  the  side  of  the  tube.  At 
HK  about  half  the  tube  is  cut  away  for  a  length  of  10  cm. 
and  the  opening  is  covered  with  thin  aluminium.  This  is  to 
allow  the  rays  to  pass  into  the  tube  and  ionize  the  gas  in  the 
part  AD.  Pass  a  slow  steady  current  of  air  through  AB  from 
a  gasometer,  by  weighting  the  gasometer  so  as  to  secure  con- 
stant pressure.  Maintain  this  stream  of  air  as  steady  as 
possible.  Place  a  plug  of  glass  wool  at  A  so  as  to  remove  all 
dust  particles  from  the  air  as  it  enters  AB.  As  the  air  passes 
AD  it  is  ionized  and  this  ionized  air  is  carried  past  the  elec- 
trodes a,  b,  c  and  d.  The  farther  the  air  proceeds  along  the  tube 
after  passing  D  the  longer  time  have  the  ions  to  recombine, 
and  the  number  of  ions  at  each  electrode  should  be  less  than  at 
the  preceding  one.  Connect  the  outer  tube  to  the  battery  as 
shown  and  connect  the  electrode  a  to  the  electrometer  and 
measure  the  saturation  current.  Then  connect  b  to  the  elec- 
trometer and  measure  the  current,  and  do  this  in  turn  for 
each  of  the  electrodes.  In  each  case  all  the  electrodes  but  the 
one  connected  to  the  electrometer  must  be  connected  to  the 


RECOMBINATION    OF   IONS  93 

tube  so  there  is  no  field  between  the  tube  and  these  electrodes 
to  extract  the  ions  before  they  reach  the  particular  one  under 
consideration.  The  current  should  be  proportional  to  the 
number  of  ions  per  c.c.  in  the  gas  as  it  passes  each  electrode. 
The  decrease  in  this  current  as  we  pass  from  one  electrode  to 
the  next  is  a  measure  of  the  decrease  in  the  number  of  ions 
in  the  time  taken  for  the  air  .to  pass  from  one  electrode  to  the 
next.  This  time  taken  by  the  air  to  pass  between  two  elec- 
trodes may  be  determined  by  observing  the  total  volume  of 
air  which  leaves  the  gasometer  per  second  and  measuring  the 
cross-section  of  the  tube  AB.  By  altering  the  speed  and 
repeating  the  measurements  the  conductivity  corresponding  to 
other  intervals  of  time  may  be  obtained.  Knowing  the  exact 
position  of  the  volume  of  air  ionized  by  the  rays  determine 
the  time  taken  by  the  air  to  pass  from  this  position  to  each  elec- 
trode in  turn.  Let  these  times  be  tlt  t2,  etc.,  and  let  the  number 
of  ions  per  c.c.  at  each  electrode  be  nx,  n2,  etc.,  which  are  pro- 
portional to  the  conductivity  measured  at  each  electrode.  Plot 
a  curve  having  as  ordinates  the  conductivities  and  as  abscissae 
the  corresponding  times. 

In  equation  (2)  the  value  of  n  for  any  given  time  t  may  be 
obtained  if  N  and  a  are  known.  Supply  two  of  the  experi- 
mental values  for  n^  and  n2  along  with  the  corresponding  values 
for  /±  and  t2  in  the  equation  and  thus  obtain  two  numerical 
equations  in  N  and  a,  and  from  these  solve  for  TV  and  a  in 
relative  numbers.  Now  supply  these  numerical  values  for  N 
and  a  inequation  (2)  and  a  general  numerical  equation  between 
n  and  t  will  be  obtained.  Select  any  values  for  t  and  solve  for 
the  corresponding  values  for  n  and  then  plot  these  theoretical 
values  for  n  and  t  on  the  same  scale  as  the  experimental  curve 
already  plotted  and  compare  these  two  curves.  The  experi- 
mental curve  and  the  theoretical  one  should  agree  very  closely, 
thus  verifying  the  theoretical  curve  and  the  formula  from 
which  it  was  obtained. 

65.  Diffusion  of  Ions,  (a)  Theory. — If  two  gases,  such  as 
hydrogen  and  carbon  dioxide  for  instance,  are  brought  together 
and  left  in  contact  and  undisturbed  they  gradually  intermingle 


94  RONTGEN    RAYS 

or  diffuse  into  one  another  owing  to  the  motion  of  the  mole- 
cules. The  same  thing  occurs  in  the  case  of  an  ionized  gas. 
The  ions  are  in  motion,  and  if  there  is  an  excess  of  ions  in  one 
part  of  the  gas  they  will  diffuse  to  the  other  part.  If  the  ion- 
ized gas  is  in  a  closed  vessel  or  between  plate  electrodes  the  ions 
will  diffuse  to  the  sides  of  the  vessel  or  the  plates  and  disappear 
from  the  gas.  Sometimes  in  a  very  confined  space  the  diffusion 
to  the  walls  or  electrodes  is  an  even  more  important  factor 
than  recombination  in  causing  the  loss  of  ions,  but  in  a  large 
volume  of  gas  the  diffusion  is  much  less  important  in  com- 
parison with  recombination.  A  study  of  this  diffusion  of  ions 
throws  considerable  light  on  the  conditions  existing  in  an 
ionized  gas  and  leads  to  some  important  results. 

The  rate  at  which  the  ions  diffuse  depends  upon  the  nature 
of  the  gas  in  which  they  exist,  as  is  to  be  expected,  just  as 
the  rates  at  which  different  gases  diffuse  are  different.  The 
diffusion  of  ions  through  the  heavy  gases  is  slower  than 
through  the  lighter  ones.  In  a  very  dry  gas  the  negative 
ions  always  diffuse  faster  than  the  positive  ions,  while  if  the 
gas  contains  considerable  moisture  the  rates  of  diffusion  of  the 
positive  and  negative  ions  are  much  more  nearly  equal.  The 
rates  of  diffusion  of  the  positive  and  negative  ions  differ  more 
in  some  gases  than  in  others.  This  difference  between  the 
positive  and  negative  ions  explains  the  phenomenon  which  is 
so  often  observed  that  if  an  ionized  gas  containing  equal  num- 
bers of  positive  and  negative  ions  be  passed  through  a  metal 
tube  it  will  emerge  positively  charged.  The  negative  ions 
diffuse  faster  to  the  sides  of  the  tube  than  do  the  positive  ions, 
and  consequently  more  negative  than  positive  ions  are  lost 
and  the  gas  emerges  with  an  excess  of  positive  electricity. 

The  rate  at  which  ions  diffuse  through  gases  is  much  slower 
than  the  rate  of  interdiffusion  of  ordinary  gases.  ^For  in- 
stance, air  and  carbon  dioxide  interdiffuse  over  five  times  as 
fast  as  the  positive  ion  diffuses  through  moist  carbon  dioxide. 
Heavy  gases  diffuse  slower  than  light  ones,  and  since  the  rate 
of  diffusion  of  both  the  positive  and  negative  ions  in  carbon 
dioxide  is  so  small  compared  with  the  rate  of  diffusion  of 


DIFFUSION    OF   IONS 


95 


carbon  dioxide  into  air,  the  natural  conclusion  is  that  the  mass 
of  the  ion  in  carbon  dioxide  is  large  compared  with  the  mass 
of  the  molecule.  This  is  true  in  the  case  of  other  gases  also. 

These  facts  have  led  to  the  theory  that  both  the  positive  and 
negative  ions  at  ordinary  pressures  consist  of  a  cluster  of 
molecules  surrounding  a  charged  nucleus.  lonization  is  be- 
lieved to  consist  in  the  separation  of  a  negative  electron  from 
the  neutral  molecule  and  then  this  charged  electron  becomes 
loaded  with  a  cluster  of  molecules  of  gas  and  forms  the  nega- 
tive ion  under  ordinary  conditions.  The  positive  ion,  on  the 
other  hand,  consists  to  begin  with  of  the  molecule  deprived  of 
the  electron  and  then  a  cluster  of  molecules  forms  about  this 
positively  charged  centre.  This  theory  accounts  for  the  fact 
that  the  positive  and  negative  ions  diffuse  more  nearly  at  the 
same  rate  in  moist  than  in  dry  gases,  for  in  dry  gases  the 
negative  ion  is  smaller,  but  in  moist  gases  it  becomes  more 
loaded  up  with  moisture  than  does  the  positive  ion,  and  con- 
sequently its'  rate  of  diffusion  decreases  more  than  that  of  the 
positive  ion. 

This  theory  is  supported  by  the  fact  that  as  the  pressure  of 
the  gas  is  lowered  the  coefficient  of  diffusion  of  the  negative 
ion  increases  faster  than  that  of  the  positive.  It  has  been 
shown  by  J.  J.  Thomson  and  by  Townsend  that  at  low  pressures 
the  negative  ion  is  identical  with  the  electron.  These  facts 
point  to  the  conclusion  that  the  negative 
ion  at  low  pressures  loses  the  cluster  of 
molecules  attached  to  the  electron. 

The  calculation  of  the  rate  of  diffu- 
sion in  most  cases  becomes  somewhat 
complicated,  but  as  an  illustration  one 
of  the  simplest  cases  will  be  considered 
here  namely  the  diffusion  of  the  ions 
between  two  parallel  plates.  Let  P  and 
Q  (Fig.  39)  be  two  parallel  plates  FIG 

in    the    ionized    gas    and    consider    the 

passage  of  the  ions  by  diffusion  across  a  small  rectangular 
space  of  thickness  dx  and  cross  section  ABCD  of  unit  area. 


96  RONTGEN    RAYS 

Let  n  be  the  number  of  positive  and  m  the  number  of  negative 
ions  per  c.c.  Then  the  number  of  positive  ions  that  pass  across 
the  face  ABCD  per  second  by  diffusion  will  be  proportional 
to  dn/dx  and  will  be  equal  to  D  dn/dx,  where  D  is  a  constant. 
Therefore  the  rate  of  increase  of  the  positive  ions  in  the  small 
volume  will  be  d/dx  of  (D  dn/d.r)  which  equals  D  d2n/dx2. 
If  q  is  the  number  of  ions  per  c.c.  produced  in  the  gas  per 
second  by  the  ionizing  source  then  the  total  rate  of  change  in 
the  number  of  ions  per  second  which  is  dn/dt  will  be  made 
up  of  three  factors,  namely,  rate  of  production  q,  rate  of 
diffusion  D  d2n/dx*,  and  rate  of  recombination  anm. 


-rt,      £  T^ 

Therefore  —  —  q  -f  D  -j-^  —  anm, 

since  q  and  D  d2n/dxz  both  tend  to  increase  the  number  in  the 
small  volume  while  anm  tends  to  decrease  the  number.  When 
a  steady  state  is  reached  dn/dt  =  o,  since  there  is  no  change, 
and  therefore 


r 

q  -\-  D  —j—^  —  canm  =  O. 
ax 

If  the  plates  are  very  close  together  so  that  the  loss  in  the 
number  of  ions  due  to  diffusion  is  much  more  marked  in 
comparison  with  that  due  to  recombination,  then  the  r.erm 
anm  may  be  neglected  and  the  equation  becomes 


If  however  the  plates  are  several  centimeters  apart  so  that  the 
loss  by  diffusion  to  the  plates  is  very  small  compared  with 
the  loss  by  recombination,  then  the  term  D  dzn/dx2  drops  out 
and  the  equation  reduces  to  the  one  we  considered  in  con- 
nection with  recombination  alone.  The  quantity  D  is  called 
the  coefficient  of  diffusion  and  is  one  of  the  important  con- 
stants in  connection  with  ionization. 


DIFFUSION    OF    IONS 


97 


(b)  Experimental  Determination.  —  The  experimental  de- 
termination of  this  coefficient  D  in  absolute  measure  involves 
some  difficulties  and  requires  very  careful  manipulation.  The 
following  comparatively  simple  experiment  will  serve  simply 
to  illustrate  the  rate  of  diffusion  of  ions  without  determin- 
ing D  in  absolute  measure.  AB  (Fig.  40)  is  a  brass  tube 
about  50  cm.  long  and  4  cm.  diameter.  CD  is  an  aluminium 
window  about  10  cm.  long  to  allow  the  rays  to  enter  the 
tube.  Obtain  some  brass  tubing  about  3  mm.  internal  diam- 
eter and  of  as  uniform  bore  as  possible.  Cut  a  dozen  lengths 
exactly  10  cm.  long.  In  two  brass  disks  which  tightly  fit 
inside  the  large  tube  AB  bore  a  dozen  holes  symmetrically 
arranged  around  the  centre  of  the  disks,  and  into  these  holes 
solder  the  small  tubes  as  shown  at  ab.  Make  also  another  set 


A 

£         f 

,* 

VJU 

/• 

\   \   !  /    '•  a  '  '         '  '  ^ 

£ARTH 


FIG.  40. 


of  a  dozen  small  tubes  exactly  the  same  as  above,  only  make 
them  i  cm.  long  instead  of  10  cm.  Make  the  disks  to  fit  the 
tube  AB  accurately,  so  that  no  gas  can  pass  between  the  disk 
and  the  larger  tube,  but  not  rigidly  fastened  so  they  may  be 
removed.  EF  is  a  brass  tube  about  20  cm.  long  and  just  large 
enough  to  fit  tightly  into  the  end  of  AB,  so  that  it  may  be 
removed  at  will.  H  is  a  central  electrode  about  10  cm.  long, 
insulated  by  an  ebonite  plug  K  as  shown.  The  tube  EF  and 
the  electrode  H  are  connected  up  in  the  usual  manner  for 
measurement  as  indicated. 

Place  the  long  set  of  small  tubes  at  B  and  pass  a  steady 


90  RONTGEN    RAYS 

stream  of  air  from  a  constant  pressure  gasometer  through  the 
system  entering  at  A.  The  gas  is  ionized  by  the  rays  before  it 
enters  the  small  tubes  and  on  its  passage  through  them  loses 
some  of  its  ions  by  diffusion  to  the  walls  of  these  tubes.  Charge 
the  tube  EF  positively  and  the  positive  ions  which  escape  from 
the  tubes  ab  will  be  driven  to  the  electrode  H,  and  the  rate  at 
which  H  charges  up  will  be  proportional  to  this  number  of 
positive  ions.  Measure  this  rate  of  charge  by  the  electrometer. 
Then  charge  EF  negatively,  and  similarly  the  charge  per  second 
acquired  by  H  will  be  proportional  to  the  number  of  negative 
ions  which  escape  from  the  tubes  ab.  Repeat  these  measure- 
ments several  times,  keeping  the  stream  of  air  constant.  Note 
that  more  positive  ions  escape  than  negative  ions.  Now  replace 
this  set  of  tubes  by  the  set  of  short  ones  and  repeat  the  meas- 
urements, being  careful  to  maintain  the  stream  of  air  of  exactly 
the  same  velocity  as  before.  In  the  second  case  more  of  both 
positive  and  negative  ions  should  escape,  as  they  have  a  shorter 
time  to  diffuse  while  passing  through  the  tubes.  Both  sets  of 
experiments  should  be  repeated,  using  a  stream  of  air  of  a 
different  velocity.  If  the  velocity  is  slower  fewer  ions  should 
escape,  while  with  greater  velocity  more  should  escape  to  be 
driven  to  the  electrode  H. 

The  air  before  passing  into  AB  should  be  thoroughly  dried 
and  freed  from  dust  particles  by  passing  it  through  drying 
agents  and  through  a  plug  of  glass  wool.  These  experiments 
may  be  repeated,  using  moist  air,  when  it  should  be  found 
that  the  difference  between  the  diffusion  of  the  positive  and 
negative  ions  is  less  than  in  the  dry  air. 

66.  Mobility  of  Ions. — When  an  electric  field  is  applied  to 
an  ionized  gas  the  charged  ions  move  under  the  influence  of  this 
field.  The  velocity  with  which  they  move  is  of  importance  in 
many  cases.  The  velocity  of  the  ions  under  a  potential  gradi- 
ent of  one  volt  per  centimeter  is  generally  termed  the  mobility 
of  the  ions.  As  in  the  case  of  diffusion  the  velocities  of  the 
positive  and  negative  ions  are  not  the  same.  The  velocity 
of  the  positive  ion  in  any  given  gas  is  always  less  than  that  of 
the  negative  ion. 


VELOCITY   OF   IONS  99 

The  mobility  of  ions  depends  upon  the  nature  of  the  gas  in 
which  they  are  produced,  being  greater  in  light  than  in  heavy 
gases.  In  dry  air,  for  instance,  the  velocities  of  the  positive 
and  negative  ions  respectively  are  1.36  and  1.87  cm.  per  second 
for  a  potential  gradient  of  I  volt  per  centimeter,  while  in 
hydrogen  the  corresponding  values  are  6.70  and  7.95  cm.  per 
second. 

The  presence  of  moisture  has  a  marked  effect  on  the  mobility 
of  ions,  causing,  as  a  rule,  a  diminution  of  velocity,  especially 
in  the  case  of  the  negative  ion.  This  is  more  marked  in  some 
gases  than  in  others.  This  diminution  of  velocity  in  the  pres- 
ence of  moisture  is  a  further  indication,  especially  in  the  case 
of  the  negative  ion,  of  an  increase  in  size  by  becoming  loaded 
with  moisture. 

The  velocity  of  the  ions  also  varies  with  the  pressure  of  the 
gas,  as  would  be  expected,  for  the  fewer  the  number  of  mole- 
cules present  the  less  would  the  ions  be  retarded  by  collisions 
with  the  molecules,  and  they  would  thus  have  greater  oppor- 
tunity to  gain  in  speed  under  the  same  electric  field.  This 
velocity  is  found  to  be  very  approximately  inversely  propor- 
tional to  the  pressure  of  the  gas.  At  low  pressures  however 
an  unusual  increase  in  velocity  of  the  negative  ion  takes  place, 
greater  than  would  be  expected  from  the  mere  change  in  pres- 
sure, which  indicates  a  diminution  in  size  of  the  ion  at  the 
low  pressure  caused  by  the  ion  shedding  the  molecules  which 
adhere  to  it. 

There  are  several  experimental  methods  which  have  been 
used  to  measure  the  velocity  of  ions  under  different  conditions. 
Some    of    these    are    applicable 
only  to  a  determination  of  the 
sum  of  the  velocities  of  the  posi- 
I      tive    and    negative    ions,    while 
others  are  only  suitable  for  spe- 
cial   cases.      By    the    following  FlG  4I 
method,  which  is  due  to  Zeleny. 

the  velocity  of  either  the  positive  or  negative  ions  may  be 
measured  independently  and  under  different  conditions.     The 


100  RONTGEN    RAYS 

theory  of  the  method  is  as  follows:  Suppose  AB  and  CD 
(Fig.  41)  represent  two  coaxial  metal  cylinders  between  which 
a  stream  of  air  is  passed  at  a  constant  velocity  in  a  direction 
parallel  to  the  axis.  A  narrow,  well-defined  section  mn  of 
the  gas  as  it  passes  along  is  ionized  by  Rontgen  rays.  A 
constant  difference  of  potential  is  maintained  between  the  outer 
and  inner  cylinders.  The  ions  will  therefore  have  two  com- 
ponent velocities,  one  due  to  the  motion  of  the  gas  parallel  to 
the  axis  of  the  tube  and  the  other  at  right  angles  to  the  axis 
due  to  the  electric  field.  If  the  outer  cylinder  be  positive  with 
regard  to  the  inner,  then  the  path  of  a  positive  ion  starting 
from  a  point  m  will  be  represented  by  the  line  md.  If  the 
potential  be  reversed  this  will  represent  the  path  of  a  negative 
ion. 

Let  a  and  b  be  the  radii  of  the  inner  and  outer  cylinders 
respectively,  E  the  potential  difference  between  them  and  X 
the  electric  field  along  the  radius  at  any  point  p  at  a  distance 
r  from  the  axis.  Then  by  the  ordinary  formula  for  the  electric 
field  between  two  coaxial  cylinders, 


r  log  - 

&e  a 

Let  v  be  the  velocity  of  the  ion  due  to  unit  electric  field 
and  v±  its  velocity  due  to  the  field  X.    Then 

Ev 

V^  =  XV  =  —        — 7  . 

r  log  - 
&ea 

Let  V2  be  the  velocity  of  the  air  current  parallel  to  the  axis 
and  dx  the  small  distance  passed  over  in  this  direction  in  the 
time  dt;  let  dr  be  the  distance  traversed  by  the  ion  along  the 
radius  at  right  angles  to  the  axis  in  the  same  time.  Then, 
since  the  distance  traversed  is  proportional  to  the  velocity, 

dx  ^  v2 

dr      v, ' 


VELOCITY   OF    IONS  IOI 


rlog  - 

be 


Therefore  dx  =  —  v»dr  =  —  ~ 

v^   2  Ev 

l°Z*a  C 
x  =      „       I 


and 


where  .#•  is  the  total  horizontal  distance  between  mn  and  rf. 

Since  the  gas  is  travelling  with  a  velocity  v2  cm.  per  second 
the  volume  of  gas  passing  between  the  cylinders  per  second 

2irrv<,dr,  which   may  be   denoted  by   W  ',  and 

. 
therefore 

/*  w 

\    rvjtr  =  —  . 

J,  2TT 


*   W 
Therefore  Jr___._ 


and  v—  --  F~  -log  -. 

&« 


The  volume  PF  may  be  determined  by  observing  the  number  of 
cubic  centimeters  of  gas  which  leave  the  gasometer  per  second. 
The  distance  .r  is  the  maximum  horizontal  distance  traversed 
by  an  ion  starting  from  the  inside  surface  of  the  outer  cylinder 
and  this  distance  depends  upon  the  potential  E  ;  the  smaller  the 
value  of  E  the  farther  will  the  ion  travel  in  a  horizontal  direc- 
tion. An  ion  which  starts  from  a  point  nearer  the  axis  than 
the  inside  surface  of  AB  will  not  have  a  chance  to  travel  so 
far,  as  it  will  be  pulled  in  to  CD  sooner,  and  therefore  only  the 
ions  which  start  from  the  inner  surface  will  reach  the  distance 
x.  To  determine  .v  experimentally  the  inner  cylinder  is  divided 
at  d  by  a  narrow  space  so  that  the  two  parts  are  insulated, 
the  right-hand  one  being  connected  to  an  electrometer  and 
the  left-hand  part  to  earth.  If  E  is  great  enough  all  the  ions 


102 


RONTGEN   RAYS 


will  be  pulled  into  the  part  Cd,  but  if  E  is  gradually  diminished 
a  point  will  be  reached  when  the  ions  just  reach  the  part  dD, 
and  this  is  indicated  by  the  movement  of  the  electrometer 
needle.  Thus  the  distance  x  is  known  and  the  corresponding 
value  of  E  may  be  determined  and  W,  a  and  b  may  all  be 


EARTH 


FIG.  42. 

measured  and  the  value  of  v  calculated  from  equation  (i). 
In  practice  some  small  corrections  have  to  be  made  on  ac- 
count of  the  diffusion  of  the  ions  and  their  mutual  repulsions, 
etc.,  the  details  of  which  may  be  found  in  the  original  papers. 
The  arrangement  of  the  apparatus  for  this  determination  is 
shown  in  Fig.  42.  AC  and  DB  are  brass  tubes  of  the  same 
diameter  of  about  5  or  6  cm.  The  part  CD  is  a  thin-walled 
aluminium  tube  of  exactly  the  same  internal  diameter  as  the 
brass  tubes.  This  is  to  allow  the  rays  to  pass  through.  These 
are  fitted  end  to  end  and  the  joints  made  gas-tight  by  fitting  a 
ring  tightly  around  the  joints  and  waxing  them  on  the  out- 
side. The  total  length  of  these  combined  tubes  should  be  about 
a  meter.  EF  is  a  thin  aluminium  tube  I  cm.  in  diameter,  co- 
axial with  AB  and  closed  by  conical  shaped  ends  and  divided 
at  the  point  H  into  two  parts,  so  that  the  space  separating  them 
is  0.5  mm.  These  two  separate  portions  are  supported  by  two 
stout  brass  rods,  R  and  Rlt  passing  through  ebonite  plugs  fitting 
into  the  wall  of  the  tube  AB.  At  a  distance  of  4  or  5  cm.  from 


VELOCITY   OF   IONS  103 

H  a  narrow  cross  section  of  the  air  is  ionized  by  the  rays 
from  the  bulb.  The  width  of  this  section  is  regulated  by  two 
thick  parallel  lead  screens,  5"  and  5^,  in  which  narrow  slits 
are  cut  about  2  or  3  mm.  in  width.  The  electrode  Rv  is  con- 
nected to  the  electrometer,  while  R  is  connected  to  earth  and 
the  outer  tube  is  connected  to  one  pole  of  a  battery,  while  the 
other  pole  is  to  earth.  The  steady  stream  of  gas  from  the  gas- 
ometer enters  the  tube  at  A.  Start  the  stream  of  gas  through 
the  tube  at  a  slow  rate  of  only  a  few  centimeters  per  second. 
Start  the  Rontgen  ray  bulb  and  put  on  a  fairly  large  positive 
potential  E  on  the  outer  tube  from  the  battery  M.  Test 
whether  the  electrometer  receives  any  charge.  If  it  does  in- 
crease the  potential  on  AB  till  it  receives  no  charge.  Then, 
keeping  the  stream  of  gas  perfectly  constant,  carefully  adjust 
this  potential  until  the  electrometer  is  just  on  the  point  of 
receiving  a  charge.  Observe  this  potential  Et  which  will  be 
the  required  value  of  E  for  the  positive  ion.  Now  reverse  the 
battery  and  make  AB  negative  and  again  carefully  adjust  the 
potential  and  let  it  be  E2.  Then 

velocity  of  positive  ion  _  E^ 
velocity  of  negative  ion      Et ' 

Find  from  this  the  ratio  of  the  velocities  of  the  positive  and 
negative  ions.  Determine  W  from  the  gasometer  and  measure 
x,  a  and  b  and  calculate  from  equation  ( I )  the  absolute  velocity 
for  both  the  positive  and  negative  ions. 

Change  the  speed  of  the  air  current  and  repeat  the  experi- 
ments, and  from  these  observations  determine  the  velocities. 
This  should  be  done  for  several  different  speeds  of  air  cur- 
rents. These  observations  should  give  very  approximately 
correct  values  without  applying  the  various  corrections  men- 
tioned above  and  will  give  a  very  good  idea  of  the  velocities 
of  ions  without  making  the  experiment  too  elaborate. 

In  these  experiments  the  air  should  be  thoroughly  dried 
before  entering  the  tube  by  passing  it  through  drying  agents. 
Afterwards  moist  air  may  be  used  to  test  the  effect  of  moisture 


RONTGKN    RAYS 


on  the  velocities.  The  velocities  in  different  gases  should  also 
be  determined  if  a  sufficient  supply  of  other  gases  is  obtainable. 
67.  lonization  by  Collision. — In  a  former  paragraph  (§55) 
the  relation  between  the  ionization  current  and  potential  in  a 
gas  at  atmospheric  pressure  was  considered,  and  there  it  was 
shown  that  when  the  voltage  reached  a  certain  value  all  the 
ions  present  were  swept  to  the  electrodes  and  no  further  in- 
crease of  current  could  take  place  for  an  increase  of  voltage, 
and  the  saturation  current  was  obtained.  At  low  pressures 
however  in  the  neighborhood  of  i  mm.  of  mercury  the  relation 
between  current  and  potential  presents  a  new  phenomenon 
which  does  not  appear  at  the  higher  pressures. 

This  may  be  studied  by  means  of  the  ionization  chamber 
shown  in  Fig.  37.  Adjust  the  distance  between  the  plates  P 
and  P'  to  about  2  cm.  Adjust  the  width  of  the  cone  of  rays 
so  that  they  do  not  touch  the  plates  as  they  pass  through  the 
gas.  Exhaust  the  vessel  to  a  pressure  of  about  I  mm.  Con- 
nect S  and  S'  to  the  electrometer  and  to  the  negative  pole  of 
a  battery  respectively  in  the  usual  way.  Apply  a  potential  of 
only  a  few  volts  to  S'  and  measure  the  current  between  the 
plates  when  the  rays  are  acting.  Increase  the  potential  by  a 

few  volts  and  measure  the 
current.  Continue  this  until 
a  potential  of  about  300  volts 
is  reached,  keeping  the  pres- 
sure constant.  Plot  the  curve 
for  current  and  voltage.  Re- 
peat this  for  two  or  three 
different  pressures  below  a 
millimeter  and  plot  the  curves. 
The  curves  obtained  should 
present  the  general  form 
shown  in  Fig.  43.  It  will  be 
observed  that  for  compara- 
tively low  voltages  the  first 

part  of  the  curve  up  to  a  point  A  is  of  the  same  form  as  the 
saturation  curve  at  atmospheric  pressure,  but  when  the  voltage 


IONS  fey  COLLISION  105 

is  increased  beyond  a  certain  amount  the  current  begins  to 
increase  again,  at  first  slowly  and  then  very  rapidly.  The 
voltage  at  which  this  increase  begins  will  depend  upon  the 
pressure  of  the  gas.  The  increase  of  the  current  beyond  the 
point  A  must  be  caused  by  an  increase  in  the  number  of  ions 
due  to  some  cause  other  than  the  original  ionizing  source,  for 
we  have  already  seen  that  Rontgen  rays  produce  fewer  ions 
at  low  pressures  than  at  high  pressures. 

Now  we  know  that  if  a  stream  of  rapidly  moving  cathode 
ray  particles  or  electrons  be  allowed  to  pass  between  two  elec- 
trodes in  a  cathode  ray  tube  an  ionization  current  is  produced 
between  the  electrodes,  showing  that  the  rapidly  moving  elec- 
trons or  ions  must  have  ionized  the  molecules  of  the  gas. 
To  ionize  a  molecule  a  certain  definite  amount  of  energy  is 
required.  A  moving  ion  possesses  kinetic  energy,  and  if  the 
velocity  is  sufficiently  great  it  will  possess  sufficient  energy 
to  ionize  a  molecule  if  it  collides  with  it.  Thus,  if  the  moving 
ions  acquire  sufficient  energy  fresh  ions  will  be  produced.  The 
kinetic  energy  of  the  ion  depends  upon  the  velocity  and  the 
velocity  depends  upon  the  electric  field  and  upon  the  chance 
the  ion  has  of  acquiring  speed  among  the  molecules  of  the  gas. 
At  atmospheric  pressure  the  molecules  are  so  numerous  that 
the  ion  is  not  able  between  two  collisions  to  acquire  sufficient 
velocity  under  ordinary  electric  fields  to  ionize  the  molecules 
by  collision  with  them,  but  at  low  pressures  the  molecules  are 
so  few  in  number  and  far  apart  that  the  speed  of  the  ion  has  a 
chance  to  increase  sufficiently  under  smaller  fields  between  col- 
lisions to  ionize  the  molecules  when  it  strikes  them.  Under 
these  circumstances  then  the  few  ions  produced  by  the  Rontgen 
rays  acquire  sufficient  velocity  under  the  influence  of  the  elec- 
tric field  to  produce  fresh  ions  by  collision  with  the  molecules 
and  these  ions  in  time  produce  more  and  the  number  increases 
very  rapidly  with  the  increase  of  voltage.  This  increase  is 
therefore  only  observed  at  the  lower  pressures  under  ordinary 
conditions,  for  at  the  higher  pressures  the  voltage  necessary  to 
produce  the  required  velocity  is  so  very  large,  being  at  atmos- 
pheric pressure  about  30,000  volts.  For  ordinary  electric  fields 


106  RONTGEN   RAYS 

this  production  of  ions  by  collision  is  only  observed  for  pres- 
sures below  about  30  mm.  of  mercury. 

This  theory  of  ionization  by  collision  furnishes  a  very  satis- 
factory explanation  of  the  electric  spark  through  a  gas  at 
atmospheric  pressure.  There  are  always  a  few  ions  existing 
in  gases  which  can  be  detected  only  by  sensitive  instruments. 
If  a  voltage  high  enough  to  produce  a  spark  is  established 
between  two  points  the  few  ions  existing  in  the  field  will 
acquire  a  velocity  sufficient  to  ionize  the  molecules  against 
which  they  strike;  these  new  ions  in  turn  will  produce  more 
ions  by  collision,  and  so  the  number  increases  very  rapidly, 
until  there  are  enough  ions  to  carry  a  current  and  this  current 
is  the  electric  spark.  Since  the  gas  is  at  atmospheric  pres- 
sure the  voltage  required  is  very  large,  but  with  decrease  of 
pressure  the  ions  acquire  velocity  more  easily  and  the  potential 
necessary  to  produce  the  discharge  decreases. 


CHAPTER  VI. 
OTHER    SOURCES    OF    IONIZATION. 

68.  Ultra-violet  Light. — Up    to   the   present   we  have,  for 
the  sake  of  simplicity,  confined  our  attention  chiefly  to  the 
ionization  produced  by  Rontgen  rays.     Cathode,  Lenard  and 
Canal  rays  all  produce  ions,  but  these  rays  are  limited  in  their 
application  to  gases  at  low  pressure.     There  are  still  other 
sources   from  which  ions  may  be  produced.     If  ultra-violet 
light  rays  fall  upon  the  clean  surface  of  an  insulated  plate  of 
zinc  which  is  negatively  charged  the  plate  will  lose  its  charge, 
while  if  the  plate  be  uncharged  to  begin  with  it  will  acquire 
a  positive  charge.     If  the  plate  is  positively  charged  to  begin 
with,  no  loss  of  charge  takes  place.     These  effects,  which  are 
called  photo-electric  effects,  have  been  shown  to  be  due  to  the 
liberation  of  negative  corpuscles,  or  electrons,  from  the  metal 
by  the  action  of  the  ultra-violet  light.    In  a  gas  at  atmospheric 
pressure  the  electrons  become  attached  to  the  molecules  and 
act  as  ordinary  negative  ions  produced  by  other  agencies,  but 
if  produced  in  a  gas  at  very  low  pressure  they  do  not  become 
loaded  with  the  molecules  of  the  gas  and  consequently  have 
a  small  mass. 

69.  Method  of  Producing  Ultra-violet  Light. — Light  rich  in 
ultra-violet  rays  may  for  general  uses  be  obtained  from  various 
sources,  such  as  an  ordinary  arc-lamp  or  the  spark  from  an 
induction  coil  between  zinc,  cad- 
mium   or    iron    terminals.     For 

making  definite  measurements 
however  a  constant  source  is 
desirable,  and  the  following 


method  will  be  found  to  produce  FIG.  44. 

very   satisfactory  results.     Two 

iron  wires  a  and  b  (Fig.  44)  about  I  or  1.5  mm.  in  diameter 

are  fitted  into  screws  which  are  supported  by  a  wooden  frame 


107 


IOS  OTHER    SOURCES    OF    IONIZATION 

ABC.  By  means  of  these  screws  the  distance  apart  of  the 
ends  of  the  wires  may  be  adjusted.  These  wires  are  made  the 
terminals  of  the  secondary  of  an  induction  coil  in  parallel  with 
which  there  should  be  a  capacity  of  three  or  four  good  sized 
Leyden  jars.  An  alternating  current  is  sent  through  the 
primary  and  this  current  should  be  adjusted  so  that  with  a 
spark  length  of  from  4  to  6  mm.  the  ends  of  the  iron  wires 
a  and  b  should  become  white  hot  but  not  hot  enough  to  melt. 
If  this  spark  is  run  at  regular  time  intervals  during  a  series  of 
measurements  and  kept  carefully  adjusted  it  will  give  a  very 
satisfactory  source  for  a  considerable  time. 

In  cases  where  greater  accuracy  is  desired  a  more  reliable 
though  a  more  elaborate  spark  arrangement  shown  in  Fig.  45 
may  be  used.  AB  is  a  brass  tube  about  3  cm.  in  diameter 
and  12  cm.  long.  C  and  D  are  two  side  tubes  about  1.5  cm. 
in  diameter  and  6  or  8  cm.  long.  The  iron  wires  a  and  b  pass 
through  glass  tubes  t  and  t  to  prevent  sparking  between  the 
wires  and  the  brass  tube.  These  'glass  tubes  in  turn  pass 
through  ebonite  plugs  in  the  ends  of  the  tubes.  The  joints  are 

made  gas-tight  by  sealing 
wax.  The  end  of  the 
larger  tube  at  A  is  closed 
by  a  quartz  window  sealed 
to  the  end  of  the  tube  by 
wax.  When  any  windows 
or  lenses  are  required 
through  which  the  ultra- 


'J1  J>yy  violet  light  is  to  pass  quartz 

must    of    course    be    used 

FlG<  4'5  as    ordinary   glass   absorbs 

the  ultra-violet  rays.     The 

iron  terminals  are  kept  in  an  atmosphere  of  pure  hydro- 
gen by  passing  a  continuous  slow  stream  of  hydrogen, 
which  has  been  thoroughly  dried,  through  this  apparatus, 
entering  at  E  and  emerging  at  O.  This  hydrogen  should  also 
be  thoroughly  free  from  oxygen,  for  if  any  oxygen  be  pres- 
ent water  vapor  is  formed  by  the  action  of  the  spark,  and  this 


ULTRA-VIOLET   LIGHT 


109 


hinders  the  passage  of  the  ultra-violet  rays.  Such  an  arrange- 
ment, which  is  due  to  Varley,  will  be  found  very  constant  over 
considerable  intervals. 

With  either  of  these  forms  of  spark  to  secure  constancy  the 
current  in  the  induction  coil  should  be  kept  constant,  the 
spark  length  should  be  maintained  at  a  constant  length  and  the 
spark  should  be  run  at  regular  time  intervals,  as  far  as 
possible. 

70.  lonization  by  Ultra-violet  Light. — An  ionization  cham- 
ber suitable  for  testing  the  ionization  produced  by  ultra-violet 
light  is  shown  in  Fig.  46,  in  which  AB  is  a  brass  cylinder,  of 
which  the  length  AB  is  about  6  cm.  and  the  diameter  AC  10 
cm.  An  opening  ab,  5  cm.  in  diameter,  in  the  end  is  closed 
by  a  quartz  plate  P  sealed  to  the  brass  by  wax.  D  is  a  side 
tube  2  cm.  in  diameter  and  5  cm.  long,  in  the  end  of  which  an 
ebonite  plug  fits.  Through  this  plug  passes  a  light  brass  rod  K 
on  the  end  of  which  is  a  metal  circular  frame  ff  6  cm.  in 
diameter,  across  which  is  stretched  a  fine  copper  grating  con- 
sisting of  very  fine  wires  not 
more  than  0.3  mm.  in  diameter. 
This  grating  should  be  situated 
not  more  than  about  a  centi- 
meter from  the  quartz  window. 
A  zinc  electrode  H,  5  cm.  in 
diameter,  is  supported  by  a  brass 
rod  which  passes  out  through  an 
ebonite  plug,  which  in  turn  fits 
into  a  brass  tube  EF,  soldered 
into  the  end  of  the  large  cylinder 


FIG.  46. 


and  about  5.5  cm.  in  diameter.  This  zinc  plate  may  be  adjusted 
anywhere  from  5  to  10  mm.  from  the  wire  grating.  Before  be- 
ing placed  in  position  this  plate  should  be  thoroughly  cleaned 
with  emery  paper  so  that  it  may  present  a  bright,  clean  surface 
on  which  the  ultra-violet  light  is  to  fall.  All  the  joints  between 
the  ebonite  and  metal,  etc.,  are  made  gas-tight  by  waxing.  The 
wire  grating  is  connected  through  the  brass  rod  K  to  the  elec- 
trometer while  the  zinc  plate  H  is  connected  to  the  negative 
pole  of  the  battery,  the  other  pole  being  to  earth. 


110  OTHER    SOURCES    OF   IONIZATION 

Set  up  the  spark  apparatus  so  that  the  source  of  ultra-violet 
light  is  at  the  principal  focus  of  a  convex  lens  of  quartz,  in 
order  to  obtain  a  parallel  beam  of  ultra-violet  light  after  pass- 
ing through  the  lens.  Allow  this  beam  to  pass  into  the  ioniza- 
tion  chamber  through  the  window  P  and  fall  normally  upon 
the  zinc  electrode  H. 

Start  with  the  air  in  AB  at  atmospheric  pressure.  Run  the 
spark  a  few  times  to  secure  regularity  of  action  and  then 
measure  the  current  produced  between  the  plate  and  the  grating 
in  the  usual  manner  for  different  voltages  on  the  plate  H,  and 
plotjfehe  usual  voltage-current  curve.  Note  that  with  even  fairly 
high  voltages  it  does  not  reach  perfect  saturation.  Reverse 
the  potential,  making  H  positive  and  test  the  current.  Note 
that  in  this  case  no  current  is  produced,  whereas  if  Rontgen 
rays  were  used  an  equal  current  would  be  produced.  This  indi- 
cates that  in  the  case  of  ultra-violet  light  the  current  is  in  only 
one  direction  and  must  be  carried  by  negative  ions  only,  for 
when  H  is  positive  the  grating  receives  no  charge.  Reduce 
the  pressure  a  few  centimeters  and  determine  the  voltage- 
current  curve  with  H  charged  negatively.  Repeat  this  for 
various  pressures  below  an  atmosphere.  Note  at  the  lower 
pressures  that  the  curves  show  the  second  rise  corresponding  to 
the  production  of  fresh  ions  by  collision.  If,  at  these  pres- 
sures, the  potential  on  H  which  will  cause  ions  to  be  produced 
by  collision  be  changed  to  the  same  positive  potential  a  current 
should  be  observed,  for  under  these  conditions  both  positive 
and  negative  ions  are  produced  by  the  collisions  of  the  negative 
electrons  with  the  molecules,  and  therefore  ions  of  both  sign 
are  present  and  a  current  may  be  obtained  in  both  directions. 
Test  this  carefully  at  different  pressures. 

Replace  the  air  by  other  gases  in  turn  and  repeat  the  experi- 
ments performed  in  air.  Compare  carefully  the  voltage-cur- 
rent curves  obtained  in  the  different  gases. 

Several  other  substances  besides  zinc  exhibit  this  photo- 
electric effect,  such  as  potassium,  sodium,  lithium,  magnesium, 
etc.  Copper,  platinum,  silver  and  a  few  other  metals  exhibit 
it  to  a  less  extent. 


PHOTO-ELECTRIC    FATIGUE  III 

To  obtain  this  photo-electric  effect  the  surface  of  the  zinc 
or  other  metal  employed  should  be  as  clean  and  well  polished 
as  possible.  If  the  surface  becomes  tarnished  the  effect  is 
greatly  diminished  or  even  entirely  destroyed. 

71.  Photo-electric  Fatigue. — The  rate  of  emission  of  elec- 
trons from  some  metals  is  much  greater  at  the  beginning  of 
the  exposure  to  the  ultra-violet  light  than  after  the  light  has 
fallen  upon  the  surface  for  some  time.     This  effect  is  usually 
known  as  photo-electric  fatigue.     The  cause  of  this  phenome- 
non has  not  yet  been  thoroughly  determined,  as  it  appears  to 
depend  upon  a  variety  of  conditions,  such  as  the  nature  of  the 
met^__emrjloy£d,Jthe  nature  of  the  gas  surrounding  the  metal, 
the  pressure  of  the  gas  and  even  the  quality  of  the  ultra-violet 
light  used.     It  may  easily  be  observed  in  the  case  of  zinc  in 
air  at  atmospheric  pressure.     Starting  with  a  perfectly  clean 
zinc   surface  in  air  at  atmospheric  pressure  test  the  photo- 
electric effect  at  intervals  extending  over  some  time  and  note 
that  the  effect  diminishes  appreciably  after  the  plate  has  been 
exposed  to  the  light  for  some  time. 

72.  Incandescent  Solids. — Many  years  before  the  ionization 
theory  of  gases  was  advanced  it  was  knowrn  that  a  red-hot  or 
white-hot  metal  caused  the  air  about  it  to  conduct  electricity. 
If  a  metal  electrode  be  placed  near  to  a  metal  wire  and  the 
latter  be  heated  until  it  begins  to  glow  a  current  through  the 
gas  will  be  produced  and  the  electrode  will  receive  a  charge. 
The  charge  received  by  the  electrode  and  the  current  depend 
upon  several  conditions,  such  as  the  temperature  of  the  wire, 
the  pressure  and  nature  of  the  gas  surrounding  it  and  also 
the  material  of  the  wire.     The  behavior  of  hot  metals  and 
the  gases  surrounding  them  is  very  irregular,  but  it  has  been 
found  that  the  gases  surrounding  these  hot  metals  present  all 
the  characteristic  properties  of  ionized  gases.     In  general  the 
results  of  experiments  show  that  metals  and  carbon  heated  to 
incandescence  in  high  vacua  give  off  negatively  charged  car- 
riers.    The  ratio  of  the  charge  to  the  mass  of  these  carriers 
has  been  shown  to  be  the  same  as  for  the  cathode  ray1  particles, 
and  the  electron  liberated  by  ultra-violet  light  at  low  pressures. 


112  OTHER   SOURCES   OF   IONIZATION 

This,  along  with  other  considerations,  has  led  to  the  theory 
that  these  negative  corpuscles  are  distributed  throughout  the 
volume  of  metals  at  all  temperatures,  but  when  the  metals  are 
heated  to  incandescence  the  corpuscles  acquire  sufficient  energy 
to  escape  into  the  surrounding  space. 

At  lower  temperatures  and  higher  pressures  positive  car- 
riers are  given  off  from  some  metals  when  heated.  Although 
this  question  has  been  investigated  to  a  considerable  extent, 
yet  the  phenomena  seem  so  complicated  that  no  very  definite 
explanation  has  as  yet  been  arrived  at.  The  results  seem  to 
indicate  that  the  positive  carriers  are  due  in  some  way  to  a 
disintegration  of  the  hot  metal  or  to  some  chemical  action  and 
that  the  positive  carriers  are  large  compared  with  the  negative 
corpuscles. 

The  general  action  of  heated  solids  "in  this  regard  may  be 
illustrated  by  the  following  experiments. 

73.  Heated  Platinum. — The  form  of  apparatus  shown  in 
Fig.  47  will  be  found  convenient  for  the  study  of  heated 
platinum.  AB  is  a  glass  tube  about  4  cm.  in  diameter  and  10 
cm.  long.  A  fine  platinum  wire  ab  is  fastened  at  the  ends,  by 
welding,  to  two  heavier  platinum  wires  cd,  which  are  sealed 
through  the  glass  at  the  ends  of  the  tube.  PQRS  is  an  alumin- 
ium cylinder  from  2  to  3  cm.  diameter  surrounding  the  plati- 
num wire  and  supported  by  an  aluminium  rod  H,  which  is 
fastened  to  a  platinum  wire  sealed  through  the  end  of  the  tube 
T.  A  couple  of  narrow  slits  should  be  cut  in  this  cylinder 
opposite  to  one  another  so  the  wire  may  be  seen  as  it  is 
gradually  heated  up. 

Connect  through  a  suitable  adjustable  resistance  7^  and  key 
K  to  the  wires  c  and  d  a  battery  H1  of  large  storage  cells 
capable  of  producing  a  steady  current  of  several  amperes  for 
the  purpose  of  heating  the  wire  ab.  Connect  the  wire  ab 
through  one  of  the  terminals  c  to  one  pole  of  a  set  of  small 
accumulators  H2  and  the  cylinder  PR  to  the  electrometer  in  the 
usual  way  as  shown  in  the  diagram.  Start  with  the  vessel 
filled  with  air  at  atmospheric  pressure  and  gradually  increase 
the  current  through  ab  from  the  battery  H1  until  ab  begins  to 


HEATED    PLATINUM 


glow.  Make  the  pole  of  H2  connected  to  ab  the  positive  one 
and  when  the  wire  begins  to  glow  observe  that  the  cylinder 
receives  a  positive  charge,  indicating  the  passage  of  positive 
electricity  from  ab  to  the  cylinder.  Make  ab  negative  and  note 
that  the  cylinder  receives  no  charge.  This  indicates  that  only 
positive  electrification  is  given  off  at  this  temperature.  Gradu- 
ally increase  the  temperature  of  the  wire  and  note  that  the 
positive  charge  received  by  the  cylinder  when  ab  is  positive 
gradually  increases  until  a  maximum  is  reached  at  about  the 
temperature  of  a  yellow  heat,  and  as  the  temperature  is  further 
increased  the  charge  received  by  PR  diminishes. 

Starting  again  with  the  wire  at  a  temperature  of  about  a 
yellow  heat,  or  a  little  above  this,  gradually  exhaust  the  air 
from  the  vessel  and  observe  the  current  between  ab  and  the 
cylinder  at  the  different  stages.  Note  that  for  a  consider- 


§•#-] 

u  * 

n  2        £AKTH 


TO  PUMP 


EARTH 


FIG.  47. 

able  range  of  pressure  below  atmospheric  pressure  no  very 
great  change  of  current  takes  place  until  a  pressure  somewhat 
below  a  millimeter  is  reached,  when  the  gain  of  positive  charge 
by  the  cylinder  begins  to  diminish  and  finally  when  a  very 

9 


OTHER   SOURCES   OF   IONIZATION 


low  pressure  is  reached  the  charge  received  by  PR  changes  sign 
and  finally  reaches  quite  a  high  negative  value,  showing  that 
the  hot  wire  at  this  low  pressure  gives  off  negative  electricity. 
In  order  of  course  to  detect  this  the  potential  on  ab  must  be 
negative  and  this  should  be  carefully  experimented  on  about 
the  reversal  point. 

When  the  low  pressure,  at  which  the  negative  electricity  is 
given  off,  is  reached  keep  the  pressure  constant  and  measure 
the  ionization  current  for  different  potentials  between  ab  and 
PR  and  plot  the  current-voltage  curve  in  the  usual  manner, 
which  will  be  found  to  resemble  the  usual  form  of  saturation 
curve. 

If  the  platinum  wire  has  not  been  heated  before  it  will  be 
found  necessary  to  keep  continuously  pumping  out  the  vessel 
at  the  low  pressures  to  maintain  the  pressure  constant,  as  the 
heating  of  the  wire  causes  it  to  give  off  occluded  gases  which 
increase  the  pressure  in  the  vessel.  Similar  measurements 
may  be  made  with  a  fine  copper  wire  in  place  of  the  platinum. 

74.  Heated  Carbon. — Similar  phenomena  at 
low  pressures  may  be  observed  in  the  case  of 
"  heated  carbon.     The  carbon  filament  from  a 
small  incandescent  lamp  may  be  utilized  for 
this    purpose.     The   thick   filament    from    an 
8-volt  lamp  will  be  found  suitable,  and  it  may 
be  used  without  removing  it  from  its  fixtures 
in  the  lamp  by  breaking  off  the  top  of  the 
lamp  A   (Fig.  48)  and  then  carefully  joining 
on  a  wide  glass  tube  T  which  has  first  been 
drawn  down  to  a  smaller  diameter  and  joined 
on  to  the  tip  of  the  lamp.     After  the  joint  is 
made  it  may  be  carefully  blown  out  to  a  larger 
size  so  as  to  admit  the  electrode  B,  which  sur- 
rounds the  filament  C.     This  electrode  B  con- 
sists of  an  aluminium  cylinder  which  is  sup- 
ported by  a  rod  sealed  through  the  glass  tube 
D.     On  the  other  end  of  D  a  bulb  E  is  blown  and  the  joint 
between  this  bulb  E  and  the  tube  T  is  made  with  sealing  wax 
as  described  in  §  31. 


IONIZATION"  FROM  FLAMES  115 

The  discharge  of  electricity  by  the  carbon  filament  must  be 
tested  only  at  the  low  pressures,  for  in  the  presence  of  oxygen 
the  carbon  oxidizes  at  the  high  temperatures  and  would  con- 
sequently be  destroyed  at  the  higher  pressures.  Test  the  dis- 
charge from  the  carbon  at  the  low  pressures  as  to  whether  it 
is  positive  or  negative.  Determine  the  current-voltage  curve. 

Substitute  hydrogen  for  air  in  the  vessels  and  repeat  the 
experiments  with  both  platinum  and  carbon  and  compare  the 
results  with  those  obtained  with  air. 

In  some  of  the  above  experiments  it  will  be  found  that  the 
current  between  the  wire  and  the  electrode  is  so  large  that  it 
may  easily  be  measured  by  means  of  a  sensitive  galvanometer. 
In  these  cases  it  will  be  found  more  convenient  to  use  a  gal- 
vanometer instead  of  the  electrometer. 

75.  lonization  from  Flames. — Flames  form  another  very 
common  source  of  ions.  Gases  surrounding  flames  contain 
ions  and  will  conduct  electricity.  If  two  electrodes  are  placed 
some  distance  apart  in  an  ordinary  Bunsen  flame  quite  an 
appreciable  current  is  observed  which  may  be  measured  by  a 
galvanometer.  If  the  air  surrounding  such  a  flame  be  drawn 
away  from  the  flame  it  is  found  to  be  still  conducting,  as  a 
gas  ionized  in  any  other  manner  would  be.  Ions  are  produced 
by  a  considerable  variety  of  flames,  but  it  is  necessary  for  the 
flames  to  be  of  a  comparatively  high  temperature.  Low 
temperature  flames  do  not  produce  ions.  The  ions  produced 
by  flames  appear  to  be  much  larger  than  ions  produced  in 
other  ways,  for  their  velocity  has  been  measured  and  found  to 
be  much  less  than  that  of  other  ions. 

The  experimental  determination  of  the  electrical  conditions 
in  flames  is  somewhat  complicated  by  the  fact  that  in  order 
to  make  measurements  metal  electrodes  must  be  used  and  when 
placed  in  the  flame  they  become  incandescent  and  give  off 
electricity  themselves,  and  it  is  difficult  to  separate  this  effect 
from  the  effects  due  to  the  flame  alone.  The  following  simple 
experiments  will  however  serve  to  illustrate  the  electrical  prop- 
erties of  flames. 

Place  an  ordinary  coal  gas  flame  between  two  parallel  metal 


Il6  OTHER    SOURCES    OF    IONIZATION 

plates  so  that  there  is  a  space  of  half  a  centimeter  or  so  on 
each  side  of  the  flame  between  it  and  the  plates.  Charge  one 
of  these  plates  to  a  high  positive  potential  and  the  other  to  a 
high  negative  potential  by  a  set  of  accumulators.  Observe  that 
the  outer  part  of  the  flame  where  the  most  combustion  takes 
place  is  drawn  towards  the  negative  plate  while  the  interior 
or  cooler  part  of  the  flame  is  attracted  to  the  positive  plate, 
showing  that  the  hotter  part  is  positively  charged  while  the 
cooler  part  is  negatively  electrified. 

In  a  Bunsen  flame  place  the  ends  of  two  platinum  wires 
and  connect  one  of  these  through  a  sensitive  D'Afsonval  gal- 
vanometer to  one  pole  of  a  storage  battery  of  only  a  few 
volts  and  the  other  electrode  to  the  other  pole.  Observe  the 
current  as  measured  by  the  galvanometer.  Test  this  current 
for  various  positions  of  the  electrodes  in  the  flame  and  note 
that  the  magnitude  and  direction  of  the  current  depends  greatly 
upon  the  relative  positions  of  the  electrodes. 

Place  the  platinum  electrodes  some  distance  apart  in  the 
flame  and  note  the  current.  Drop  a  few  crystals  of  any  of 
the  ordinary  salts,  which  volatilize  when  heated,  into  the  flame 
between  the  electrodes  and  observe  the  sudden  increase  of  cur- 
rent produced.  The  same  effect  will  be  observed  if  a  solution 
of  any  of  the  salts  be  introduced  by  spraying  into  the  flame. 

Place  a  Bunsen  flame  at  the  end  of  a  glass  or  brass  tube  3  or 
4  cm.  in  diameter  and  30  or  40  cm.  long  and  allow  the  other 
end  of  the  tube  to  open  into  a  fairly  large  sized  electroscope. 
By  an  aspirator  attached  to  another  opening  in  the  electroscope 
draw  a  slow  current  of  air  past  the  flame  and  into  the  electro- 
scope. Charge  up  the  gold  leaf  of  the  electroscope  and  ob- 
serve that  as  the  air  current  is  passing  the  charge  quickly  dis- 
appears, showing  that  the  air  surrounding  the  flame  is  con- 
ducting and  it  retains  its  conductivity  for  a  time  after  being 
•removed  from  the  neighborhood  of  the  flame  just  as  an  ionized 
gas  will  do. 

This  conductivity  of  flames  is  often  made  use  of  in  dis- 
charging an  electrified  body.  If  for  instance  any  insulation 
has  become  charged  up  on  the  surface  it  will  sometimes  take 


IONIZATION    FROM    FLAMES  Iiy 

a  long  time  for  the  charge  to  leak  away,  and  in  the  meantime 
it  will  cause  troublesome  electrostatic  disturbances.  To  get 
rid  of  this  charge  on  the  surface  it  is  sufficient  to  simply  pass 
a  Bunsen  flame  quickly  over  the  surface  a  few  times  (see  §  15), 
when  the  conductivity  of  the  flame  will  allow  the  charge  to  be 
conducted  away  through  the  flame.  A  flame  furnishes  a 
convenient  conductor  in  this  case,  as  good  contact  can  be 
made  over  the  whole  surface  of  the  insulation  so  as  to  com- 
pletely get  rid  of  the  charge. 

As  pointed  out  previously  (§15)  the  presence  of  flames  in 
the  neighborhood  of  insulated  conductors  is  to  be  carefully 
avoided  or  otherwise  the  conductors  will  lose  their  charge 
through  the  conductivity  imparted  to  the  air  by  the  flames. 


CHAPTER  VII. 
IONS    AS    NUCLEI. 

76.  General  Phenomena. — For  some  years  before  the  ioniza- 
tion  theory  of  gases  was  propounded  it  was  known  that  if  dust 
particles  were  present  in  a  damp  gas  the  water  vapor  would 
condense  around  these  dust  particles  as  nuclei  when  a  sudden 
expansion  of  the  gas  took  place.     It  had  also  been  observed 
that  if  a  highly  charged  electrode,  such  as  the  terminal  of  an 
influence  machine  or  induction  coil,  were  placed  near  a  trans- 
parent steam  jet  a  marked  change  in  the  jet  occurred  during 
the  escape  of  electricity  from  the  electrode.     The  steam  con- 
densed into  fine  drops,  as  could  be  shown  by  the  increased 
opacity  of  the  jet  if  its  shadow  were  cast  on  a  screen. 

After  the  discovery  of  Rontgen  rays  it  was  shown  that  if  a 
beam  of  the  rays  were  allowed  to  fall  upon  the  steam  jet  con- 
densation took  place.  Since  it  had  been  proved  that  dust 
particles  acted  as  nuclei  of  condensation  some  maintained  that 
the  condensation  of  the  steam  jet  in  the  cases  of  the  charged 
electrode  and  Rontgen  rays  was  also  due  to  dust  while  others 
explained  the  effect  on  the  steam  jet  as  due  to  the  presence 
of  ions.  In  1897  and  later  C.  T.  R.  Wilson  proved  by  a 
series  of  valuable  experiments  that  ions  do  act  as  nuclei  on 
which  water  vapor  will  condense  when  moist  air  is  suddenly 
cooled  by  expansion. 

77.  Expansion  Apparatus. — The  later  and  improved  form 
of  apparatus  designed  and  used  by  Wilson  is  shown  in  Fig. 
49.     A  is  the  vessel  in  which  the  expansion  takes  place.     It 
consists  of  a  heavy  glass  cylinder  ab  about  16  or  18  cm,  in 
diameter  and  about  8  cm.  in  height.     The  ends  of  the  cylinder 
are  ground  flat  so  that  the  brass  plates  cd  and  ef  fit  tightly 
against  the  ends.     Between  these  plates  and  the  ends  of  the 
cylinder  thin  rubber  washers  should  be  placed  and  the  two' 
plates  drawn  tightly  together  against  the  ends  by  six  or  eight 

118 


EXPANSION  APPARATUS  119 

brass  rods  rr  acting  as  bolts  with  nuts.  If  this  does  not  make 
the  vessel  air-tight  the  joints  may  be  waxed.  From  the  middle 
of  the  lower  plate  ef  a  brass  tube  T,  about  7  cm.  in  diameter 
and  25  cm.  long,  leads.  The  lower  end  of  this  is  closed  by  a 
large  rubber  stopper  5\  This  tube  also  contains  a  closely  fitting 
piston  P  which  consists  of  a  light  brass  tube  with  a  hemi- 


FIG.  49. 

spherical  top.  This  piston  should  be  made  to  slide  easily  inside 
the  tube.  Through  the  stopper  S  another  glass  or  brass  tube 
leads  into  an  air-tight  chamber  B  and  from  this  leads  another 
tube  T2  to  a  large  glass  receiver  R,  which  is  connected  to  an 
air-pump.  For  this  purpose  a  large  glass  bottle  with  two  out- 
lets may  be  used.  The  opening  of  the  tube  T2  is  closed  by  a 
rubber  stopper  S1  attached  to  the  end  of  a  rod  rx  which  passes 
out  through  a  closely  fitting  tube  in  the  bottom  of  the  chamber 
B.  A  piece  of  rubber  tubing  t  should  fit  tightly  over  the  end 
of  the  tube  and  the  rod  r±  so  that  the  rod  may  be  moved 


120  IONS   AS    NUCLEI 

through  the  tube  without  allowing  air  to  enter  B.  The  stopper 
Sl  and  attached  rod  are  held  tightly  against  the  end  of  the 
tube  T2  by  a  stiff  spiral  spring.  From  the  upper  chamber  A 
another  small  glass  tube  T3  leads  to  a  manometer  M  by  which 
the  pressure  in  A  may  be  measured  when  the  tap  h  is  open. 
R!  and  R2  are  two  mercury  reservoirs  connected  by  a  rubber 
tube  by  which  the  pressure  in  A  may  be  regulated.  The  tap  h^ 
closes  the  tube.  The  shaded  part  in  the  tube  T  is  filled  with 
water  which  serves  to  keep  the  air  in  A  saturated  as  well  as  to 
make  a  flexible  air-tight  joint  between  the  piston  P  and  the 
tube  T. 

Now  suppose  that  the  mouth  of  the  tube  T2  is  closed  by 
Si  and  R  is  partially  exhausted  by  the  air-pump.  Suppose  the 
air  in  both  A  and  B  is  at  atmospheric  pressure.  If  h±  be  closed 
and  h  opened  and  R2  be  lowered  slightly,  then  the  pressure  in  A 
will  be  slightly  relieved  and  the  piston  P  will  rise  a  little  until 
the  pressure  in  A  and  B  balance.  Then  if  5\  be  suddenly  with- 
drawn from  the  end  of  T2  the  air  in  B  will  suddenly  rush  into 
R  and  thereby  lower  the  pressure  in  B  to  a  point  considerably 
less  than  that  in  A,  and  the  piston  P  will  be  suddenly  forced 
down  against  S.  This  results  therefore  in  a  sudden  expansion 
of  the  air  in  A.  The  more  rapidly  this  expansion  takes  place 
the  better,  and  therefore  the  rod  ri  should  be  attached  to  a 
trigger  arrangement  with  a  strong  spring  so  as  to  obtain  a  very 
sudden  release  and  consequently  a  very  sudden  opening  of  T2. 

78.  Production  of  Clouds. — Close  the  valve  St  and  the  tap 
hlt  open  the  taps  h2  and  h  and  by  adjusting  R2  raise  the  piston 
P  only  a  short  distance  in  Ihe  tube  T,  so  that  only  a  small 
expansion  will  take  place  when  P  is  forced  down.  Close  the 
tap  h ;  partially  exhaust  the  reservoir  R  and  suddenly  open  v$\. 
Observe  the  dense  cloud  formed  in  A.  To  observe  this  to  the 
best  advantage  render  the  drops  visible  by  brightly  illuminat- 
ing the  space  in  A  by  a  strong  beam  of  light  from  an  arc 
lamp,  concentrated  by  means  of  a  glass  convex  lens.  This 
lens  not  only  serves  to  concentrate  the  light,  but  also  cuts  out 
any  ultra-violet  light  which  would  be  apt  to  affect  the  results. 
Observe  the  cloud  gradually  fall.  When  the  drops  have 


CLOUDS   BY   EXPANSION  121 

disappeared  repeat  the  experiment,  expanding  the  air  in  A 
again.  Repeat  this  several  times  and  observe  that  a  cloud  is 
formed  each  time,  but  that  it  gradually  becomes  less  dense 
until,  after  a  number  of  expansions,  practically  no  cloud  is 
formed.  These  clouds  are  due  to  the  dust  particles  in  the  air 
around  which  the  water  vapor  condenses  and  the  dust  is  carried 
down  with  the  drops  and  therefore  after  several  expansions 
the  air  is  finally  freed  from  dust.  After  the  air  is  thus  prac- 
tically freed  from  nuclei  no  cloud  is  formed  if  the  amount  of 
expansion  is  below  a  certain  limit. 

Even  in  air  freed  from  dust  particles  clouds  may  be  formed 
by  expansion  under  certain  conditions.  The  formation  of  a 
cloud  in  dust-free  air  depends  upon  the  extent  of  the  expansion. 
Wilson  has  shown  that  if  v±  is  the  volume  of  the  air  in  A 
before  expansion  and  vz  the  volume  after  expansion  then  if 
vz/Vj_  is  less  than  1.25  no  condensation  occurs  in  dust-free  air; 
if  vjv^  is  between  1.25  and  1.38  a  few  drops  may  be  observed 
but  as  soon  as  z>2A'i  becomes  greater  than  1.38  a  dense  cloud  is 
formed  on  expansion  even  if  no  dust  be  present.  Test  these 
facts  carefully  by  expansion.  The  amount  of  expansion  de- 
pends upon  the  height  to  which  P  is  raised,  and  since  the  vol- 
ume is  inversely  proportional  to  the  pressure  the  ratio  of  the 
volumes  before  and  after  expansion  may  be  determined  by  ob- 
serving the  pressure  as  indicated  by  the  manometer  M  before 
and  after  expansion. 

Completely  free  the  space  A  from  dust  by  repeated  expan- 
sions. Then  adjust  by  trial  the  height  of  the  piston  P  so  that 
the  ratio  of  v2/v±  will  be  less  than  1.25  and  observe  on  expan- 
sion whether  any  cloud  is  formed.  By  careful  adjustment 
gradually  increase  this  ratio  and  observe  the  result  as  re- 
gards condensation.  By  a  series  of  trials  verify  the  results 
stated  above. 

79.  Ions  as  Nuclei. — If  the  gas  in  A  be  acted  upon  by  an 
ionizing  agent,  such  as  Rontgen  rays  a  change  occurs.  Pass  a 
horizontal  beam  of  Rontgen  rays  through  the  air  in  A  and 
repeat  the  experiments  of  the  last  paragraph,  and  observe  that 
for  values  of  the  ratio  v^/v^  below  1.25  no  cloud  is  formed  as 


122  IONS   AS    NUCLEI 

before.  But  between  the  values  1.25  and  1.38  a  dense  cloud 
is  formed  when  the  rays  are  acting  where  only  a  few  drops 
were  observed  before  when  no  rays  were  acting.  This  indi- 
cates that  the  ions  act  as  nuclei  on  which  the  water  vapor 
condenses. 

That  the  formation  of  the  cloud  in  this  instance  is  due  to  the 
presence  of  the  ions  and  not  to  some  other  possible  action  of 
the  rays  may  be  easily  proved  by  the  following  experiment : 
Place  in  the  chamber  A  two  insulated  parallel  plates  p  and  p^ 
as  indicated  in  the  diagram,  about  5  cm.  apart.  Establish  a 
potential  of  about  400  volts  between  these  plates.  This  electric 
field  should  remove  the  ions  as  soon  as  formed.  With  this 
large  potential  between  the  plates  pass  the  rays  through  as 
before  and  observe  that  with  the  expansion  that  would  pro- 
duce a  cloud  with  no  field  on  there  is  practically  no  more 
condensation  than  when  no  rays  acted  at  all.  Note  that  as 
soon  as  the  plates  are  disconnected  from  the  battery  the  same 
expansion  will  produce  a  dense  cloud.  This  shows  that  the 
dense  cloud  which  is  formed  by  the  rays  when  there  is  no 
field  on  is  due  to  the  presence  of  the  charged  ions,  for  when 
they  are  removed  by  the  electric  field  the  cloud  is  not  formed. 

It  can  easily  be  shown  also  that  these  drops  of  water  formed 
by  the  action  of  Rontgen  rays  are  charged,  for  if  an  electric 
field  be  applied  to  them  after  they  are  formed  they  will  move 
under  the  influence  of  the  field.  Allow  the  rays  to  ionize  the 
air  between  the  plates  in  A  as  before  but  without  any  electric 
field  on  and  produce  a  cloud  by  a  suitable  expansion.  As  coon 
as  the  sudden  commotion  in  the  air  caused  by  the  expansion 
subsides  apply  a  strong  electric  field  between  the  plates  and,  if 
the  field  is  strong  enough,  it  may  be  observed  that  some  of  the 
ions  move  towards  the  upper  plate,  while  the  others  move  more 
quickly  downwards  than  they  would  do  simply  under  the  action 
of  gravity.  If  the  field  be  reversed  the  direction  of  motion 
will  be  reversed.  This  action  of  the  electric  field  on  the  drops 
shows  that  they  carry  a  charge  and  that  there  are  charges  of 
both  signs  present,  showing  that  both  the  positive  and  negative 
ions  act  as  nuclei.  A  cloud  which  is  formed  without  the 


IONS   AS   NUCLEI  123 

action  of  an  ionizing  agent  will  not  be  affected  by  an  electric 
field 

Wilson  has  also  shown  by  means  of  a  special  form  of 
expansion  chamber  that  water  vapor  condenses  more  easily 
around  the  negative  ions  than  on  the  positive.  When  vz/v± 
exceeds  the  value  1.25  and  reaches  the  value  1.28  a  large 
amount  of  condensation  takes  place  around  the  negative  ions, 
but  very  little  condensation  takes  place  on  the  positive  ions 
until  a  value  of  1.31  is  reached.  This  is  a  confirmation  of 
the  theory  that  the  cause  of  the  greater  diminution  of  the 
velocity  and  rate  of  diffusion  of  the  negative  ion  in  moist  gases 
is  due  to  the  negative  ion  becoming  more  easily  loaded  with 
moisture  than  the  positive  ion. 

80.  Ions  from  Other  Sources  as  Nuclei. — Remove  the  plates 
p  and  />!  from  the  expansion  chamber  A  and  replace  the  plate 
cd  by  a  similar  one  which  has  an  opening  in  it  covered  with 
a  quartz  window  to  admit  ultra-violet  light.  ,  Inside  of  A  place 
a  polished  zinc  plate  some  distance  below  the  window.  Allow 
ultra-violet  light  to  pass  through  the  window  and  fall  on  the 
plate  and  repeat  the  experiments  described  in  the  first  part  of 
§  79,  using  the  ultra-violet  light  in  place  of  the  Rontgen  rays 
and  observe  the  formation  of  the  cloud,  which  indicates  that 
the  ions  produced  by  ultra-violet  light  will  act  as  nuclei  in  a 
manner  similar  to  those  produced  by  Rontgen  rays. 

Again,  remove  the  plate  cd  with  the  quartz  window  and  re- 
place it  by  another  similar  one  from  which  a  thin  platinum 
spiral  is  suspended,  with  the  two  ends  of  the  wire  passing  out 
through  small  insulating  plugs  so  that  the  wire  may  be  con- 
nected to  a  battery  by  which  it  may  be  raised  to  incandescence. 
Heat  this  platinum  wire  in  the  usual  way  by  a  storage  battery 
and  repeat  the  experiments  described  in  the  first  part  of  §  79, 
using  the  charged  nuclei  produced  by  the  hot  wire  in  the  place 
of  the  ions  produced  by  Rontgen  rays.  Note  the  values  of 
v2/v±  necessary  to  produce  the  cloud  when  the  wire  is  at 
different  temperatures.  Since  at  the  lower  temperatures  posi- 
tive ions  are  given  off  by  the  wire  while  at  the  higher  tempera- 
tures negative  ions  are  emitted,  a  difference  between  the  expan- 


124  JONS   AS    NUCLEI 

sions  necessary  to  produce  condensation  around  the  positive 
and  the  negative  ions  may  be  noted  here,  as  at  the  different 
temperatures  one  kind  predominates  over  the  other.  Carefully 
observe  this  difference. 

81.  Charge  Carried  by  an  Ion.  —  This  property  of  ions  to 
act  as  condensation  nuclei  has  been  utilized  to  determine  the 
absolute  value  of  the  charge  carried  by  an  ion.  The  method 
is  not  a  very  simple  one  and  had  better  be  left  by  the  student 
until  further  work  has  been  done  and  more  experience  gained 
in  this  subject.  The  general  principle  will  however  be  de- 
scribed here. 

When  an  expansion  takes  place  in  ionized  air  water  drops 
form  around  the  ions  and  fall  under  the  action  of  gravity.  Sir 
George  Stokes  has  shown  that  if  a  drop  of  water  of  radius  r 
falls  through  a  gas  of  viscosity  /*  then  the  velocity  v  with  which 
the  drop  falls  is  given  by  the  equation 


9     A* 

where  g  is  the  acceleration  of  gravity.  The  velocity  v  can  be 
measured  by  observing  the  rate  at  which  the  cloud  falls  under 
the  action  of  gravity,  and  since  /*  is  known  for  air  and  g  is 
also  known,  therefore  r  may  be  determined.  If  m  is  the  mass 
of  water  deposited  and  n  the  number  of  drops  per  c.c.,  then 
m  =  n  x  %7rr3,  since  the  density  of  water  is  unity.  From 
well-known  thermal  considerations  the  amount  of  water  vapor 
deposited  from  a  gas  when  a  known  expansion  occurs  can  be 
easily  calculated,  and  therefore  m  may  be  determined.  Know- 
ing m  and  r  the  number  of  drops  n,  which  is  the  same  as  the 
number  of  ions,  is  easily  calculated. 

Let  two  parallel  plates  be  placed  d  cm.  apart  in  the  expan- 
sion chamber  A  (Fig.  49),  and  let  a  potential  difference  V, 
small  compared  with  that  necessary  to  produce  saturation,  be 
applied  to  them.  Then  if  the  sum  of  the  velocities  of  the 
positive  and  negative  ions  per  unit  potential  difference  be  u  and 
the  charge  on  each  ion  be  e  the  current  i  per  square  centi- 


CHARGE    CARRIED    BY   AN    ION  125 

meter  of  cross  section  of  the  plates  is  given  by 

nuVe 


since  n  is  the  number  of  ions  per  c.c.  and  the  current  is  pro- 
portional to  the  total  charge  ne  and  the  velocity  uV  and  in- 
versely proportional  to  d.  The  value  of  n  has  been  determined 
from  the  expansion,  and  w  is  known  for  any  value  of  d  (§  66). 
The  value  of  *  can  be  measured  in  the  usual  way  by  the  elec- 
trometer and  d  and  V  can  be  measured.  Therefore  e  may  be 
calculated. 

By  the  latest  determinations  of  J.  J.  Thomson  he  has  shown 
that 

e  =  34  X  io~10  electrostatic  units. 

He  has  also  shown  that  the  charge  carried  by  the  ion  in 
hydrogen  or  oxygen  has  the  same  value  and  that  it  does  not 
depend  upon  the  source  by  which  the  ions  are  produced.  These 
results  seem  to  indicate  that  the  charge  carried  by  a  gaseous 
ion  is  the  same  under  all  circumstances,  and  it  appears  that  it 
might  be  taken  as  an  invariable  and  fundamental  unit  of 
electricity. 


PART   II. 
RADIO-ACTIVITY. 


CHAPTER  VIII. 

INTRODUCTORY  EXPERIMENTS  ON  RADIO-ACTIVE 
SUBSTANCES. 

82.  Discovery  of  Radio-activity. — The  discovery  of  Ront- 
gen  rays  and  their  close  connection  with  phosphorescence  led 
physicists  to  enquire  whether  any  natural  substances,  espe- 
cially those  which  exhibit  phosphorescence,  were  capable  of 
producing  radiations  of  a  similar  nature.  Several  substances 
were  examined  by  different  experimenters,  but  the  first  dis- 
covery of  importance  in  this  regard  was  made  by  M.  Henri 
Becquerel  in  1896,  who  found  that  the  double  sulphate  of 
uranium  and  potassium  emitted  a  radiation  which  produced  an 
effect  upon  a  photographic  plate  enclosed  in  black  paper  similar 
to  the  effect  produced  by  Rontgen  rays.  He  later  examined 
other  compounds  of  uranium  as  well  as  the  element  itself  and 
found  that  they  all  possessed  this  power.  The  extent  of  the 
action  on  the  plate  does  not  depend  upon  the  particular  com- 
bination in  which  the  uranium  occurs,  but  entirely  upon  the 
amount  of  uranium  present  in  the  compound,  which  indicates 
that  the  radiations  result  from  the  uranium  itself  and  not 
from  the  fact  of  its  association  with  other  substances. 

Although  the  connection  between  Rontgen  rays  and  phos- 
phorescence pointed  the  way  to  the  discovery  of  these  radia- 
tions from  uranium,  and  notwithstanding  that  they  were  first 
attributed  to  phosphorescence,  it  has  since  been  shown  that 
there  is  no  connection  between  these  rays  emitted  by  uranium 
and  its  phosphorescence,  for  some  compounds  which  are  not 
phosphorescent  emit  the  rays. 

127 


128  INTRODUCTORY  EXPERIMENTS 

83.  Warning. — Before  proceeding  to  use  the  various  radio- 
active substances  with  which  we  will  have  to  do  a  timely  cau- 
tion must  be  given  so  as  to  prevent  serious  difficulty  later.     In 
handling  these  radio-active  substances  the  greatest  care  must 
be  taken  not  to  spill  the  slightest  trace  of  them,  for  if  they 
become  scattered  round  the  laboratory,  even  to  the  slightest 
extent,  the  room  will  become  so  contaminated  that  after  a  time 
the  air  of  the  room  will  be  so  radio-active  that  no  fine  measure- 
ments or  accurate  work  of  this  nature  can  be  done  in  it. 

In  the  cases  of  radium,  thorium  or  actinium  great  precau- 
tions must  be  taken  to  always  keep  these  substances  tightly 
enclosed  in  an  air-tight  receptacle  even  while  working  with 
them,  for  as  we  shall  see  later  they  give  off  a  radio-active  gas 
which  will  cause  the  walls  and  other  bodies  in  the  room  to 
become  radio-active.  This  radio-activity  cannot  be  got  rid  of 
for  years.  This  latter  precaution  need  not  be  taken  in  the 
case  of  uranium  and  its  compounds. 

84.  Photographic  Action  of  Rays  from  Uranium. — Wrap  a 
photographic  plate  in  ordinary  black  paper.     Spread  a  few 
grams  of  uranium  oxide  on  a  thin  sheet  of  paper  in  a  layer 
covering  an  area  of  6  or  8  cm.  square.     Lay  two  or  three 
opaque  articles,  such  as  small  pieces  of  metal,  on  the  black 
paper  covering  the  plate  on  the  film  side  of  the  plate,  and  on 
top  of  these  pieces  of  metal  place  the  sheet  of  paper  containing 
the  uranium  oxide.     Lay  this  away  in  a  perfectly  dark  room 
for  about  twenty-four  or  thirty-six  hours.     Then  develop  the 
photographic  plate  in  the  ordinary  way,  and  observe  that  the 
plate   is   darkened   except   where   the   opaque   bodies   cast   a 
shadow.    The  uranium  thus  gives  off  a  radiation  which  affects 
the  plate  but  differs  from  light  in  the  fact  that  it  penetrates 
the  black  paper,  although  it  will  not  penetrate  the  metals.    The 
action  is  however  very  weak,  as  it  takes  several  hours  to  pro- 
duce any  impression.    If  the  uranium  oxide  and  plate  had  been 
left  for  only  an  hour  or  two  practically  no  effect  would  have 
been  observed  on  account  of  the  weak  action  of  the  rays.    The 
same  experiment  may  be  repeated  using  other  compounds  of 
uranium,  such  as  the  sulphate,  etc. 


URANIUM    RADIATIONS  129 

85.  Power  of  Uranium  Rays  to  Discharge  an  Electrified 
Body. — Using  an  electroscope  of  the  form  shown  in  Fig.  14 
cut  a  hole  about  5  cm.  square  in  the  base  plate  and  cover  this 
opening  by  a  very  thin  sheet  of  tissue  paper  simply  to  prevent 
air  currents  inside  the  electroscope.  Cut  two  sheets  of  brass 
or  zinc  15  cm.  square  and  2  or  3  mm.  thick.  In  one  of  these 
cut  a  central  square  hole  about  5  cm.  square.  To  the  other 
plate  solder  or  rivet  two  short  upright  pins,  and  in  the  plate 
from  which  the  hole  is  cut  bore  two  holes  corresponding  ex- 
actly in  position  with  the  pins  in  the  other,  so  that  the  two 
plates  may  be  fitted  together  always  in  a  definite  relative  posi- 
tion. These,  when  placed  together,  will  form  a  shallow  recep- 
tacle of  definite  area  which  may  be  used  to  hold  the  uranium 
compound. 

Place  a  few  grams  of  uranium  oxide  in  a  uniform  layer  in 
this  receptacle  and  place  it  immediately  below  the  opening  in 
the  electroscope.  Repeat  in  detail  the  experiments  described 
in  §  46,  using  the  radiations  from  uranium  in  place  of  the 
Rontgen  rays.  Note  that  the  effects  produced  are  practically 
identical  with  those  produced  by  Rontgen  rays.  The  radia- 
tions from  the  uranium  are  however  of  a  much  weaker  nature 
than  Rontgen  rays. 

Using  the  apparatus  represented  in  Fig.  28  substitute  for  the 
Rontgen  rays  a  few  grams  of  uranium  oxide  contained  in  a 
small  shallow  trough,  which  may  be  placed  either  in  the  bottom 
of  the  tube  AB  or  just  below  an  opening  cut  in  the  bottom  of 
AB.  It  is  advisable  to  cover  the  trough  with  a  sheet  of  thin 
tissue  paper  as  a  precaution  in  case  the  air  current  through 
the  system  should  accidentally  become  strong  enough  to  blow 
the  oxide  out  of  the  trough. 

Repeat  in  detail  the  experiments  described  in  §  47,  using  the 
uranium  radiations  in  place  of  the  Rontgen  rays.  In  cases 
corresponding  to  the  stoppage  of  the  Rontgen  rays  the  uranium 
will  of  course  have  to  be  removed  from  AB.  Observe  that 
the  effects  produced  by  the  uranium  radiations  are  similar  to 
those  produced  by  Rontgen  rays. 

These  experiments  indicate  clearly  that  uranium  oxide  emits 

10 


130 


INTRODUCTORY    EXPERIMENTS 


some  sort  of  a  radiation  which  produces  conductivity  in  a  gas 
similar  to  that  produced  by  Rontgen  rays.  This  conductivity 
of  the  gas  persists  for  a  time  after  removal  from  the  direct 
action  of  the  rays  and  may  be  transmitted  from  one  point  to 
another  along  with  the  air.  It  may  also  be  removed  by  mechan- 
ical or  electrical  means  in  the  same  way  as  that  produced 
by  Rontgen  rays. 

86.  lonization  Current  Produced  by  Uranium.  Apparatus. 
— Arrange  a  system  as  shown  in  Fig.  50.  AB  is  a  rectangular 
box  made  of  metal  (zinc  about  I  mm.  thick  is  suitable)  and 
should  be  about  25  cm.  each  way.  C  is  a  metal  plate  about 
16  cm.  square  resting  on  insulating  pillars,  and  is  connected  to 


M 


D 


EARTH 


EARTH 


FIG.  50. 


one  pole  of  a  battery  of  small  accumulators  as  shown.  D  is 
a  similar  metal  plate  parallel  to  C  and  suspended  by  four 
ebonite  rods  from  the  top  of  the  box.  It  is  connected  by  a 
wire,  passing  out  through  an  ebonite  plug,  to  an  electrometer 
in  the  usual  manner.  The  top  of  the  box  may  be  fitted  so  that 
it  can  be  lifted  off  and  the  whole  of  the  upper  part  of  the 
system  removed  if  desired.  One  side  of  the  box  should  be 
fitted  with  a  door  about  20  cm.  square,  which  ought  to  fit 
closely  when  closed. 


IONIZATION    PRODUCED   BY    URANIUM  13! 

Current-voltage  Curve. — Adjust  the  distance  between  the 
plates  C  and  D  to  about  a  couple  of  centimeters  by  adjusting  the 
.  height  of  C.  On  C  place  centrally  the  double  plate  receptacle, 
described  in  the  last  paragraph,  containing  a  layer  of  the  ura- 
nium oxide.  On  insulating  the  electrometer  quadrants  it  will 
be  observed  that  they  immediately  begin  to  charge  up,  indicat- 
ing the  presence  of  an  ionization  current  between  the  plates. 
Measure  this  ionization  current  in  the  usual  way  for  different 
voltages  applied  to  C.  Plot  the  current-voltage  curve  and 
observe  that  it  is  of  the  same  form  as  shown  in  Fig.  32. 

Current  and  Distance  Between  the  Plates. — Determine  the 
current-voltage  curve  for  different  distances  of  the  plates 
apart  varying  from  about  0.5  cm.  to  5  cm.  Note  that  they 
all  follow  the  same  general  form,  but  that  the  saturation  cur- 
rent increases  as  the  distance  between  the  plates  increases. 
Plot  a  curve  showing  the  relation  between  the  saturation  cur- 
rent and  the  distance  between  the  plates.  Note  that  the  curve 
is  not  a  straight  line. 

Current  and  Thickness  of  Layer  of  Material. — Place  the 
plates  a  given  distance  apart,  say  about  2  cm.,  and  place  a 
sheet  of  note  paper  in  the  bottom  of  the  receptacle  for  hold- 
ing the  oxide,  and  dust,  by  means  of  a  fine  wire  gauze,  a  uni- 
form and  very  thin  layer  of  uranium  oxide  on  the  sheet  of 
paper  and  measure  the  saturation  current.  Then  increase  the 
thickness  of  the  layer  a  little  by  dusting  some  more  oxide  on 
the  paper  and  measure  the  saturation  current  again.  Repeat 
this  a  number  of  times,  increasing  the  layer  a  little  in  thick- 
ness each  time.  Weigh  the  sheet  of  paper  before  starting  the 
experiments  and  then  weigh  it  and  the  contained  oxide  each 
time.  Observe  that  the  saturation  current  increases  with  the 
increase  in  the  amount  of  oxide  used.  At  first  with  thin  layers 
the  current  is  practically  proportional  to  the  quantity  of  oxide, 
but  after  the  layer  becomes  thicker  the  current  does  not  in- 
crease so  rapidly  with  increase  in  thickness.  Plot  a  curve 
showing  the  relation  between  the  current  and  the  quantity  of 
oxide  used. 

Current  from   Different   Compounds.- — If   different   com- 


132  INTRODUCTORY   EXPERIMENTS 

pounds  of  uranium,  such  as  the  oxide,  sulphate,  chloride,  etc., 
are  available  measure  the  saturation  currents  produced  by 
equal  quantities  of  these  compounds  under  similar  conditions 
and  compare  them.  If  it  is  convenient  samples  of  these  differ- 
ent compounds  may  be  analyzed  and  the  quantity  of  uranium 
present  in  each  determined.  If  this  is  done  it  will  be  found 
that  the  current  produced  for  a  given  weight  of  compound 
depends  not  upon  the  nature  of  the  compound  but  upon  the 
quantity  of  uranium  present  in  it. 

Current  and  Time. — Select  a  given  specimen  of  any  of  the 
radium  compounds,  say  uranium  oxide,  and  test  the  satura- 
tion current  produced  by  it  between  the  plates  at  a  given  dis- 
tance apart  and  repeat  this  test  under  exactly  the  same 
conditions  each  day  for  several  days.  It  will  be  found  that 
if  all  the  conditions  are  kept  constant  the  current  will  not 
change,  showing  that  the  radiations  do  not  change  with  time. 
This  will  be  found  to  be  still  true,  even  if  the  test  is  extended 
over  many  months. 

Current  in  Different  Gases.  —  Make  another  ionization 
chamber  on  the  same  principle  as  the  one  shown  in  Fig.  50, 
but  instead  of  the  enclosing  vessel  being  a  rectangular  box 
make  it  of  a  brass  cylinder  about  15  cm.  in  diameter  and  the 
same  height.  The  two  ends  may  be  fitted  and  then  all  the 
joints  made  gas-tight  with  wax.  The  uranium  oxide  may 
be  placed  on  the  plate  C  from  the  top  and  then  the  top  plate 
which  supports  D  put  in  place  and  waxed.  The  plates  C  and 
D  should  be  about  3  or  4  cm.  apart.  Such  a  vessel  may  be 
filled  with  any  gas  and  the  current  in  it  measured.  Fill  this 
vessel  with  any  gases  which  may  be  available  in  turn,  and 
measure  the  saturation  current  in  each  gas  at  atmospheric 
pressure  and  compare  them.  As  in  the  case  of  Rontgen  rays 
it  will  be  found  that  the  current  depends  very  much  upon  the 
nature  of  the  gas. 

Conclusions. — The  results  of  these  experiments  along  with 
others  which  might  be  performed  on  uranium  and  its  com- 
pounds show  that  the  uranium  emits  spontaneously  a  radia- 
tion without  the  aid  of  any  outside  agency  which  ionizes  a  gas 


THORIUM  133 

in  a  manner  similar  to  other  ionizing  agents  already  studie'd. 
This  radiation  belongs  to  the  uranium  itself  and  not  to  the  sub- 
stances with  which  it  is  associated.  It  depends  upon  the 
amount  of  uranium  present  in  the  compound  and  it  does  not 
deteriorate  with  time.  Uranium  and  other  bodies,  which  we 
shall  see  later  possess  similar  properties,  are  called  radio- 
active bodies  and  this  form  of  radiation  is  called  radio-activity. 
This  term  in  its  strict  application  is  applied  only  to  such  bodies 
as  are  naturally  and  permanently  radio-active,  that  is,  which 
spontaneously  emit  such  radiations,  and  not  to  such  substances 
as  may  acquire  this  property  temporarily  by  the  action  upon 
them  of  some  outside  agency. 

87.  Other  Radio-active  Substances.  Thorium. — The  dis- 
covery of  the  radio-activity  of  uranium  naturally  led  to  the 
examination  of  other  substances  to  ascertain  if  any  of  them 
possessed  similar  properties.  The  element  thorium  and  its 
compounds  were  found  to  possess  radio-active  properties,  the 
photographic  action  being  however  weaker  than  that  of  ura- 
nium, while  the  ionizing  power  was  about  equal  to  that  of 
uranium. 

Mme.  Curie  then  undertook  a  very  systematic  examination 
of  a  large  number  of  mineral  compounds  containing  uranium 
and  thorium.  Using  the  electrical  method  of  examination  she 
measured  the  current  produced  between  two  parallel  plates  by 
a  given  amount  of  each  of  the  minerals.  The  results  showed 
that  all  these  minerals  containing  thorium  or  uranium -were 
radio-active,  but  the  most  important  point  observed  was  that 
several  specimens  of  pitch-blende  (uraninite),  as  well  as  other 
minerals,  were  several  times  more  active  than  uranium  itself. 
Now  if  uranium  be  mixed  with  an  inactive  substance  the  ac- 
tivity will  be  less  than  that  of  the  uranium  alone,  owing  to  the 
fact  that  some  of  the  rays  are  absorbed  by  the  material  with 
which  the  uranium  is  mixed.  It  was  at  first  thought  that  this 
abnormal  activity  of  some  of  the  minerals  might  be  due  to  the 
particular  chemical  combination  in  which  the  uranium  existed, 
but  this  was  disproved  by  preparing  one  of  these  compounds 
artificially,  when  it  was  found  to  possess  only  the  normal 


134  INTRODUCTORY  EXPERIMENTS 

amount  of  activity  which  would  be  expected  from  the  amount 
of  uranium  it  contained.  This  led  to  the  conclusion  that  there 
must  be  some  other  and  more  active  substance  in  pitchblende. 
M.  and  Mme.  Curie  investigated  this  question  chemically  and 
found  two  new  active  bodies. 

Polonium. — The  first  of  these  substances  to  be  separated  by 
purely  chemical  means  is  much  more  active  than  uranium  and 
differs  from  it  in  the  essential  particular  that  its  activity  is  not 
constant  but  gradually  dies  away  with  time.  It  also  differs 
from  the  other  radio-active  substances  in  the  nature  of  the 
radiations  given  out,  which  will  be  discussed  later.  To  this 
substance  the  name  polonium  was  given. 

Radium. — The  other  active  substance  discovered  in  pitch- 
blende is  enormously  more  active  than  uranium.  In  its  pure 
state  it  is  about  a  million  times  more  active  and  consequently 
was  given  the  name  radium  by  the  discoverers.  Radium  is 
probably  the  most  remarkable  and  interesting  of  all  the  radio- 
active substances,  and  by  the  study  of  its  properties  an  enor- 
mous amount  of  information  has  been  obtained  in  regard  to 
the  most  remarkable  processes  going  on  in  nature  in  connec- 
tion with  these  radio-active  bodies. 

The  quantity  of  radium  existing  in  pitchblende  is  almost 
infinitesimal,  about  a  ton  of  pitchblende  containing  only  a  few 
milligrams  of  radium.  The  chemical  properties  of  radium  are 
similar  to  those  of  barium  and  it  is  separated  from  the  mineral 
pitchblende  by  the  same  process  as  is  used  in  the  separation 
of  barium. 

Radium  is  found  in  varying  quantities  in  a  number  of  min- 
erals and  in  various  parts  of  the  world,  but  the  chief  source 
at  present  known  is  in  the  pitchblende  found  in  Bohemia. 

In  practice  radium  is  not  separated  from  the  compound,  but 
is  usually  made  use  of  in  the  form  of  radium  bromide,  and 
what  is  often  called  pure  radium  is  usually  pure  radium  bro- 
mide. It  also  forms  other  compounds  such  as  the  chloride, 
sulphate,  etc. 

The  method  by  which  radium  and  polonium  were  discovered 
marks  a  great  advance  in  the  methods  of  analysis  and  of  de- 


RADIUM  135 

tecting  the  presence  of  new  bodies,  for  it  was  purely  by  their 
radio-active  properties  that  these  substances  were  discovered. 
They  were  entirely  unknown  before,  and  being  in  such  minute 
quantities  they  might  have  continued  to  escape  detection  for  a 
very  long  time,  but  their  intense  radio-active  properties  indi- 
cated their  presence  and  then  it  was  possible  to  attack  them 
by  chemical  methods  and  separate  them.  This  is  very  analo- 
gous to  the  methods  used  in  spectrum  analysis. 

Actinium. — Not  long  after  the  discovery  of  radium  another 
substance  was  found  in  some  of  the  residues  from  pitchblende 
to  which  the  name  actinium  was  given.  The  properties  of 
actinium  are  very  similar  to  those  of  thorium,  but  the  former 
is  very  much  more  active  than  the  latter. 

88.  Current  Produced  by  Other  Radio-active  Substances.— 
Obtain  small  quantities  of  thorium,  actinium  and  radium  com- 
pounds. Radium  is  obtainable  only  in  very  small  quantities 
and  is  very  expensive,  but  the  other  substances  can  be  obtained 
in  somewhat  larger  quantities.  In  a  thick  lead  or  brass  plate 
cut  a  groove  about  2  mm.  deep  by  5  mm.  wide  and  20  mm.  long. 
In  this  place  the  specimen  of  radium.  Over  this,  and  flat  on 
the  plate,  place  a  thin  sheet  of  mica  as  thin  as  is  obtainable  and 
carefully  wax  the  edges  down  so  as  to  be  gas-tight.  This 
precaution  must  be  taken  to  prevent  the  escape  of  the  gaseous 
emanation  (see  Chapter  XIII),  which  is  continually  being 
given  off  by  the  radium.  Place  this  plate  in  the  place  of  C 
(Fig.  50),  and  with  a  distance  of  about  2  cm.  between  the 
plates  measure  the  ionization  current  produced.  Determine 
the  current-voltage  curve  for  the  specimen. 

Make  similar  receptacles  for  holding  the  thorium  and  the 
actinium  compounds,  but  these  may  be  made  larger  as 
these  substances  may  be  obtained  in  larger  quantities.  These 
should  be  carefully  sealed  up  also  to  prevent  escape  of  gaseous 
emanations.  Measure  the  current  produced  by  these  speci- 
mens also  and  determine  the  current-voltage  curve  in  each  case. 

Compare  the  saturation  currents  produced  by  equal  weights 
of  these  different  specimens  with  that  produced  by  the  same 
weight  of  uranium.  Measure  the  activity  of  any  other  samples 


136  INTRODUCTORY   EXPERIMENTS 

obtainable   such   as  pitchblende,   etc.,   and   compare   them   all 
with  a  given  equal  weight  of  uranium. 

89.  Steady  Deflection  Method  of  Measuring  lonization 
Currents. — With  this  amount  of  data  in  regard  to  radio- 
activity at  our  disposal  another  and  very  useful  method  of 
measuring  ionization  currents  may  be  introduced  with  advan- 
tage at  this  stage.  The  ordinary  method  of  measuring  ioniza- 
tion currents  by  the  rate  of  movement  of  the  electrometer 
needle,  which  we  have  used  up  to  the  present,  depends  upon 
certain  conditions  being  fulfilled  and  in  some  cases  possesses 
certain  disadvantages.  In  order  that  the  rate  of  movement 
of  the  needle  may  be  proportional  to  the  ionization  current  the 
capacity  of  the  system  must  remain  constant.  If  the  current 
increases  to  a  great  extent  the  rate  of  movement  becomes  too 
rapid  to  be  measured  with  accuracy  and  the  capacity  has  to  be 
increased  to  diminish  this  rapid  rate  of  movement  to  a  readable 
amount.  This  involves  a  comparison  of  capacities  which  is 
generally  a  troublesome  task.  Again  the  rate  of  movement 
method  requires  considerable  time  to  make  the  observations, 
and  therefore  in  some  measurements  when  rapid  changes  are 
taking  place  it  is  practically  useless.  A  steady  deflection 
method  has  been  developed  by  Bronson  which  overcomes  these 
difficulties  and  has  proved  very  satisfactory  in  practice. 

Theory  of  Method. — If  the  air  between  two  plates  A  and  B 
(Fig.  50  (a)),  connected  respectively  to  a  battery  and  an 
electrometer  in  the  usual  manner,  be  ionized  by  a  radio-active 
body  placed  on  A  the  electrometer  will  continue  to  charge  up 
and  the  deflection  of  the  needle  increase  in  proportion  to  the 
voltage  to  which  the  quadrants  become  charged.  If  the  pair 
of  quadrants  connected  to  B  be  connected  to  earth  through 
a  very  large  resistance  Rr  then  some  of  the  charge  received  by 
the  quadrants  will  leak  to  earth  through  this  resistance.  The 
quadrants  will  therefore  continue  to  charge  up  and  the  de- 
flection of  the  needle  increase  until  the  rate  of  supply  of  elec- 
tricity to  the  quadrants  is  equal  to  the  loss  through  the  resist- 
ance, that  is,  the  current  through  the  resistance  R  is  equal  to 
the  ionization  current  between  the  plates  A  and  B.  When  this 


STEADY   DEFLECTION    METHOD  137 

stage  is  reached  the  deflection  of  the  needle  will  remain  con- 
stant since  the  potential  of  the  quadrants  remains  steady.  If 
the  high  resistance  R  obeys  Ohm's  law  the  current  through  R 
will  be  proportional  to  the  potential  of  the  quadrants,  and 
therefore  to  the  deflection  of  the  needle  since  the  deflection 
is  proportional  to  the  potential.  Therefore  since  the  current 
through  R  and  the  ionization  current  are  equal  this  deflection 
will  be  proportional  to  the  ionization  current. 

Since  the  ionization  currents  are  usually  so  extremely  small 
and  the  current  through  R  is  the  same  as  the  ionization  cur- 
rent the  resistance  R,  as  will  be  seen  from  the  following 
simple  calculation,  will  have  to  be  very  large.  Take  as  an 
example  the  typical  case  cited  in  §  20.  The  current  to  be 
measured  is  9.2  x  icf13  ampere.  Suppose  that  a  steady  de- 
flection of  TOO  scale  divisions  is  desired.  This  corresponds 
to  a  rise  of  potential  of  -J  of  a  volt  in  this  case  and  therefore 
the  resistance  R  required  will  be  J  -f-  (9.2  X  icf13)  which 
equals  a  resistance  of  180,000  megohms.  Ordinary  liquid  or 
carbon  resistances  of  this  order  of  magnitude  are  not  satis- 
factory for  this  purpose,  as  they  are  somewhat  unreliable. 
What  may  be  termed  an  air  resistance  has  been  found  to  be 
most  suitable.  This  consists  of  two  parallel  plates  C  and  D 
(Fig.  50  (a)  )  in  air,  on  the  lower  one  of  which  there  is  placed  a 
layer  of  radio-active  material  and  the  connections  made  as  shown 
in  the  diagram.  The  charge  received  by  the  plate  B  and  the 
electrometer  system  of  which  C  is  a  part  leaks  away  to  earth 
through  the  air  between  the  plates  C  and  D  in  consequence  of 
the  conductivity  of  this  ionized  air  and  when  the  rate  of  loss 
is  equal  to  the  rate  of  supply  a  steady  deflection  will  result. 
Such  an  air  resistance  obeys  Ohm's  law  over  a  considerable 
range  for  the  potential  acquired  by  the  plate  C  is  small,  being 
usually  only  a  fraction  of  a  volt  and  therefore  below  the  satu- 
ration voltage.  The  current  between  C  and  D  being  thus  below 
saturation  will  correspond  to  the  steep  part  of  the  current 
voltage  curve  (Fig.  32),  in  which  the  current  is  proportional 
to  the  potential.  The  steady  deflection  being  proportional  to 
the  potential  will  therefore  be  proportional  to  the  current. 


INTRODUCTORY   EXPERIMENTS 


Calibration.  —  This  proportionality  should  be  carefully  tested 
and  the  range  on  the  scale  determined  over  which  it  holds  true 
for  any  particular  setting  of  the  scale.  This  may  be  done  by 
placing  a  constant  source  of  ionization  such  as  uranium  oxide 


fART/f 


..C  ' 5 

-D  ..._         , 


EARTH 

FIG.  soa. 

on  the  plate  A  (Fig.  500)  and  applying  small  known  vol- 
tages much  below  the  saturation  voltage  where  the  current 
between  AB  is  known  to  be  proportional  to  the  voltage  and 
observing  the  steady  deflection.  The  test  can  be  carried 
further  for  greater  deflections  by  testing  several  small  speci- 
mens of  uranium  oxide  on  the  plate  A,  first  separately  and 
then  in  groups,  the  sum  of  the  currents  for  the  separate  speci- 
mens being  equal  to  the  current  for  these  specimens  tested 
in  groups. 

If  a  straight  scale  be  used  the  reading  on  the  scale  will  of 
course  not  be  proportional  to  the  angle  of  deflection  beyond  an 
angle  of  a  certain  magnitude,  but  the  proportional  reading  may 
be  extended  over  a  greater  range  by  fixing  the  scale  on  a  suit- 
able support,  so  that  instead  of  being  exactly  perpendicular  to 
the  line  joining  its  central  point  to  the  electrometer  needle 
it  may  be  turned  at  a  small  angle  to  the  perpendicular  posi- 
tion, and  it  may  even  be  bent  to  a  slight  curve  to  approach  the 
circular  form.  These  adjustments  have  to  be  made  by  trial 
in  each  particular  setting. 

Standard. — The  plates  C  and  D  should  be  made  as  nearly  as 
possible  identical,  for  on  account  of  the  contact  difference  of 
potential  between  the  plates  due  to  any  difference  in  their  sur- 
face the  needle  will  show  a  small  steady  deflection  even  when 


STEADY  DEFLECTION    METHOD  139 

there  is  no  radio-active  matter  between  A  and  B.  This  may  be 
eliminated  to  a  great  extent  by  making  the  surfaces  as  nearly 
alike  as  possible  as  regards  material,  etc. 

The  radio-active  material  on  the  plate  D  used  by  Bronson 
in  his  early  investigations  of  this  method  was  radio-tellurium 
obtained  as  a  coating  on  a  bismuth  plate  prepared  by  the 
method  of  Marckwald.  This  plate  was  covered  by  a  very  thin 
sheet  of  aluminium  foil  and  the  plate  C  made  of  aluminium  to 
eliminate  the  effect  of  any  contact  difference  of  potential.  The 
plates  C  and  D  should  be  enclosed  in  a  sealed  vessel  so  as  to 
keep  them  from  any  outside  disturbing  influences.  Radium  or 
other  constant  sources  of  radiation  contained  along  with  the 
^plates  in  a  sealed  vessel  may  however  be  used.  To  be  satis- 
factory they  must  remain  perfectly  constant. 

Advantages  of  System. — This  system  of  measuring  ioniza- 
tion  currents  has  several  advantages  over  the  rate  of  move- 
ment method.  One  decided  advantage  is  the  rapidity  with 
which  measurements  can  be  made.  The  needle  takes  up  its 
steady  position  rapidly  and  one  does  not  have  to  wait  on  the 
long  time  of  swing  and  the  return  to  zero  after  each  ob- 
servation, or  even  the  time  required  to  pass  over  a  given 
distance  to  determine  the  time  rate.  Any  change  in  the 
current  is  rapidly  indicated  by  the  needle  taking  up  a  new 
steady  position.  Readings  have  been  taken  under  some  cir- 
cumstances as  quickly  as  once  every  five  seconds.  This  is 
of  great  advantage  in  measuring  rapid  changes  of  activity 
such  as  will  be  discussed  later.  The  readings  may  also  be 
taken  over  a  large  range  of  currents  without  altering  the 
sensitiveness  of  the  electrometer  and  without  any  alteration  of 
capacity  of  the  system,  for  in  this  method  the  deflection  is 
independent  of  the  capacity.  The  potential  of  the  electrometer 
system  in  this  method  is  not  continually  increasing  at  a  time 
rate  due  to  a  constant  supply  of  electricity  as  in  the  rate 
method,  but  the  electrometer  system  constitutes  one  point  in 
what  is  practically  a  continuous  closed  circuit  and  when  the 
rate  of  supply  is  equal  to  the  loss  the  potential  of  this  point  is 
constant  and  independent  of  the  capacity.  The  needle  there- 


140  INTRODUCTORY   EXPERIMENTS 

fore  takes  up  a  steady  position  due  to  the  steady  difference  of 
potential  between  the  quadrants.  Consequently  comparative 
measurements  of  current  by  this  method  do  not  require  any 
determination  of  capacities,  even  though  the  capacities  may 
be  changed. 


CHAPTER  IX. 

COMPLEXITY    OF   RADIATIONS. 

90.  Absorption  of  Rays  from  Uranium  by  Solid  Bodies. — 

In  the  ionization  chamber  (Fig.  50)  place  the  plates  B  and  C 
about  2  cm.  apart  and  on  C  place  in  the  usual  way  a  thin  layer, 
of  not  more  than  0.5  mm.,  of  uranium  oxide.  Measure  the 
saturation  current  by  the  electrometer.  Then  place  a  thin  sheet 
of  aluminium  foil,  not  more  than  .0005  cm.  in  thickness,  over 
the  oxide  and  measure  the  saturation  current  again.  Over 
this  place  a  second  sheet  of  foil  of  the  same  thickness  and 
again  measure  the  current.  Repeat  this,  adding  a  sheet  at  a 
time  until  ten  or  a  dozen  sheets  have  been  added.  Observe 
that  for  the  first  four  or  five  sheets  (the  number  will  depend 
upon  the  thickness*  of  the  foil  used)  the  saturation  current, 
which  is  a  measure  of  the  intensity  of  the  radiation,  falls  off 
rapidly  in  a  geometrical  progression  with  the  increase  in  the 
number  of  sheets,  that  is,  according  to  an  ordinary  absorption 
law  for  any  sort  of  radiation  in  general.  When  about  four 
or  five  sheets  have  been  added  the  intensity  will  have  been 
reduced  to  probably  about  one  twentieth  of  the  original  value, 
while  the  addition  of  the  others  produces  a  very  slight  effect, 
if  any,  in  reducing  the  intensity  of  the  radiations.  After  about 
four  or  five  it  requires  a  comparatively  large  number  of  sheets 
to  produce  much  effect  in  this  regard.  It  will  be  found  that 
it  will  require  probably  from  seventy-five  to  one  hundred  thick- 
nesses of  foil  to  reduce  the  intensity  of  the  remaining  radia- 
tion to  half  its  value.  These  numbers  that  are  given  are  of 
course  only  approximate  to  serve  as  a  guide  and  will  depend 
upon  the  exact  thickness  of  foil,  etc. 

These    results    indicate    that    the    radiations    given    off    by 

*  The  thickness  may  be  determined  very  approximately  by  cutting 
exact  squares  of  foil  of  known  area  and  weighing  a  given  number  of 
them  and,  knowing  the  density,  the  thickness  may  be  determined. 

141 


142  COMPLEXITY  OF   RADIATIONS 

uranium  must  be  complex,  consisting  of  at  least  two  types  of 
rays,  one  of  which  is  capable  of  passing  through  only  a  small 
thickness  of  aluminium  foil,  while  the  other  type  is  much  more 
penetrating.  The  first  type,  or  easily  absorbable  rays,  have 
been  given  the  name  a  rays,  while  the  more  penetrating  type 
are  called  ft  rays.  The  a  rays  are  completely  cut  off  by  a 
thickness  of  about  .002  cm.  of  aluminium  and  consequently 
after  this  thickness  is  placed  over  the  uranium  only  the  ft  rays 
get  through. 

Using  the  same  thin  layer  of  uranium  oxide  cut  off  all  the 
a  rays  by  a  sheet  of  aluminium  about  .002  cm.  thick  and  then 
further  test  the  absorption  of  the  ft  rays  by  adding  increasing 
thicknesses  of  aluminium  foil  until  the  ft  rays  are  completely 
cut  off  and  note  carefully  the  decrease  of  ionization  current 
with  increase  of  thickness  of  absorbing  material.  Note  that 
it  requires  a  very  much  greater  thickness  of  aluminium  to  com- 
pletely cut  off  the  ft  rays  than  it  does  to  absorb  the  a  rays. 

From  these  measurements  it  will  be  seen  that  by  far  the 
greater  portion  of  the  total  ionization  produced  by  the  radia- 
tions from  uranium  is  produced  by  the  a  rays,  for  before  any 
aluminium  is  introduced  the  ionization  is  produced  by  the 
joint  action  of  the  a  and  ft  rays  and  we  have  seen  that  only 
about  five  thicknesses  of  aluminium  foil  completely  cut  off  the 
a  rays  but  have  little  effect  on  the  ft  rays,  and  when  the  a  -rays 
are  completely  absorbed  and  only  the  ft  rays  are  acting  the 
total  ionization  is  reduced  to  a  small  fraction  of  the  original 
when  both  a  and  ft  rays  were  acting.  This  will  be  discussed 
a  little  more  fully  in  the  following  chapter. 

Obtain  a  large  quantity  of  uranium  oxide,  as  much  as  75  or 
loo  grams  if  possible.  Make  a  receptacle  for  it  in  a  metal 
block  so  that  it  may  cover  an  area  of  about  20  or  25  sq. 
cm.  and  deep  enough  to  hold  the  quantity  available.  Measure 
the  ionization  current  produced  by  the  radiations  from  this  in 
the  manner  described  above.  Test  both  the  a  and  the  ft  radia- 
tion. Add  sheets  of  tinfoil  until  both  the  a  and  ft  rays  are 
completely  cut  off  and  the  electrometer  will  then  show  prac- 
tically no  ionization  unless  it  is  extremely  sensitive.  Now  set 


y   RAYS  143 

up  a  gold  leaf  electroscope  of  the  type  shown  in  Fig.  14. 
Measure  its  rate  of  leak  due  to  the  natural  ionization  of  the 
air  (see  Chapter  XVI).  Place  underneath  the  electroscope 
the  specimen  of  uranium  oxide  covered  with  the  tinfoil  and 
observe  that  although  the  electrometer  showed  no  ionization 
in  the  ionization  chamber  the  electroscope  shows  a  considerable 
rate- of  leak,  indicating  that  a  radiation  of  some  sort  has  pene- 
trated the  tinfoil  and  is  ionizing  the  air  in  the  electroscope. 
The  delicate  electroscope  detects  this  weak  ionization,  while 
the  electrometer  may  not  be  sufficiently  sensitive  to  do  so. 
Measure  this  rate  of  leak  and,  subtracting  the  natural  rate  of 
leak  of  the  electroscope,  the  rate  of  leak  due  to  this  radiation  is 
obtained.  Now  add  thin  sheets  of  lead,  about  0.5  mm.  thick, 
one  sheet  at  a  time,  and  after  adding  each  one  measure  the  rate 
of  leak,  that  is  the  ionization,  and  observe  that  the  radiation 
is  gradually  cut  down  in  intensity  by  the  addition  of  the  lead, 
but  that  it  requires  a  thickness  of  a  centimeter  or  more  of 
lead  to  completely  cut  off  this  radiation. 

These  results  indicate  that  in  addition  to  the  a  and  (3  rays 
given  off  by  the  uranium  there  is  an  extremely  penetrating 
radiation  emitted  which  will  pass  through  a  considerable  thick- 
ness of  a  very  dense  substance  like  lead  and  which  produces 
ionization  of  a  very  weak  character  compared  with  that  pro- 
duced by  either  the  a  or  /?  rays.  These  very  penetrating  rays 
are  called  y  rays.  The  radiation  emitted  by  uranium  is  there- 
fore complex,  consisting  of  three  types  of  rays  differing  very 
much  from  one  another  in  penetrating  power. 

91.  Rays  from  Thorium  and  Radium. — Using  specimens  of 
thorium  oxide  and  of  radium  bromide  repeat  the  experiments 
of  the  last  section  on  the  a  and  j3  rays.  As  in  the  case  of 
uranium  use  a  comparatively  thin  layer  of  each  specimen.  The 
ionization  chamber  of  Fig.  50  will  have  to  be  modified  slightly 
to  suit  these  experiments  as  these  substances  continuously  give 
off  gaseous  emanations  (Chapter  XIII),  which  produce  ioniza- 
tion independent  of  that  produced  by  the  ordinary  rays  emitted 
by  the  thorium  or  radium.  To  avoid  this  complication  the 
emanation  must  be  removed.  To  do  this  introduce  an  inlet 


144  COMPLEXITY  OF   RADIATIONS 

tube  about  8  mm.  in  diameter  in  the  upper  part  of  the  side  AM, 
and  two  or  three  outlet  tubes  in  the  side  BN,  distributed  over 
the  part  opposite  the  space  between  the  plates.  Connect  these 
outlet  tubes  in  parallel  to  a  single  tube  leading  to  a  water 
exhaust  pump  or  other  aspirator  so  as  to  draw  a  slow  steady 
current  of  air  through  the  vessel.  As  this  emanation  is  carried 
out  through  the  water  pump  special  precaution  must  be 
taken  to  prevent  the  escape  of  any  of  this  emanation  into  the 
room.  The  discharge  water  and  accompanying  air  from  the 
water  pump  should  therefore  be  led  off  by  a  special  tube  to 
the  air  outside  the  building.  If  this  precaution  is  neglected 
the  room  will  become  permanently  contaminated  by  this  emana- 
tion so  that  after  a  short  time  the  room  and  contents  will 
become  radio-active  and  no  accurate  work  of  this  kind  can 
be  carried  on  in  it.  The  nature  and  action  of  these  emana- 
tions will  be  discussed  more  in  detail  in  a  later  chapter. 

The  results  of  the  experiments  mentioned  at  the  beginning 
of  this  paragraph  will  show  that  both  thorium  and  radium 
compounds  emit  a  and  (3  rays  of  a  similar  nature  to  those 
emitted  by  uranium. 

Using  a  large  quantity,  about  75  or  100  grams,  of  thorium 
oxide  repeat  the  experiments  of  the  last  section  on  the  y  rays 
and  note  that  thorium  also  gives  off  the  penetrating  y  rays. 
The  same  experiments  should  be  performed  with  radium  bro- 
mide, but  in  this  case  a  large  quantity  of  radium  is  not  neces- 
sary even  if  it  were  available,  as  these  penetrating  y  rays  may 
quite  easily  be  detected  by  using  only  a  comparatively  small 
quantity  of  radium.  This  is  due  to  the  fact  that  radium  is  so 
very  much  more  strongly  radio-active,  weight  for  weight,  than 
either  uranium  or  thorium.  If  only  a  thin  layer  of  the  two 
latter  substances  be  used  only  a  very  small  amount  of  y  rays 
are  emitted  and  the  ionization  produced  Js  very  weak,  as  the 
y  rays  are  not  strong  ionizers.  But  if  a  large  quantity  be  used 
in  a  thick  layer,  then,  since  the  y  rays  are  very  penetrating, 
those  from  the  lower  portions  of  the  thick  layer  are  able  to 
pass  up  through  the  material  without  being  absorbed  to  any 
great  extent.  By  thus  increasing  the  thickness  of  the  layer 


MAGNETIC   DEFLECTION    OF    RAYS  145 

the  quantity  of  y  rays  is  increased  practically  in  proportion  to 
the  thickness  of  the  layer  up  to  a  certain  limit,  and  therefore 
the  ionization  is  increased  sufficiently  to  be  detected.  In  the 
case  of  radium  however  it  is  so  strongly  radio-active  that  even 
a  thin  layer  emits  a  sufficient  quantity  of  y  rays  to  be  detected. 
92.  Magnetic  Deflection  of  (3  Rays. — The  discovery  in  the 
year  1899  that  some  of  the  radiations  from  radio-active  bodies 
could  be  deviated  by  a  magnetic  field  caused  a  considerable 
advance  in  the  differentiation  of  these  rays  and  the  determina- 
tion of  their  true  nature.  It  was  found  that  the  p  rays  emitted 
by  the  various  radio-active  substances  were  affected  by  a 
magnetic  field  in  a  manner  similar  to  that  in  which  cathode 
rays  are  affected  by  a  corresponding  field.  'This  may  be  shown 
very  conveniently  in  the  following  manner:  Place  a  small 
quantity  of  radium  bromide  on  a  thick  lead  block  A  (Fig.  51), 
between  two  parallel  thick  lead  plates  BB,  which  should  be 
about  4  cm.  high  and  2  cm.  wide 
and  about  0.5  cm.  apart.  Above 

these  lead   plates   place   two   insu-         

lated   metal   plates   PP',  the   same      -j=- 
distance   apart   as   the   lead   plates      •=?=-  fl. 
and  about  7  cm.  high  and  5   cm. 
wide.     The  rays  from  the  radium 
ionize  the  gas  between  the  plates 
and  the  presence  of  the  rays  be- 
tween these  plates  may  be  detected 
by  measuring  the  ionization  current  pIG  5I< 

between    P    and    P'.      Place    this 

arrangement  between  the  poles  of  a  strong  electro-magnet 
so  that  the  magnet  field  may  be  applied  perpendicularly 
to  the  plane  of  the  paper,  that  is,  parallel  to  the  plane  of 
the  plates  BB.  The  dotted  line  in  the  diagram  represents  the 
outline  of  the  pole-pieces  of  the  magnet.  A  slow  current  of 
air  should  be  drawn  through  the  space  between  the  plates  BB 
to  prevent  the  emanation  from  diffusing  upward  between  the 
plates  P  and  P'.  Measure  the  saturation  current  between  P 
and  P'.  This  will  be  almost  wholly  due  to  the  a  and  ft  rays  as 


p> 


EARTH 


ii 


146  COMPLEXITY  OF    RADIATIONS 

the  ionization  due  to  the  y  rays  is  so  small  for  a  thin  layer 
of  material  that  it  is  negligible  in  comparison  with  that  due 
to  the  a  and  (3  rays.  Place  a  sheet  of  aluminium  .01  cm.  thick 
over  the  layer  of  active  material  to  cut  off  all  the  a  rays. 
Then  measure  the  ionization  current  due  to  the  (3  rays  alone. 
With  the  aluminium  sheet  still  covering  the  radium  apply  a 
fairly  strong  magnetic  field  and  measure  the  ionization  current. 
Observe  that  under  the  influence  of  the  magnetic  field  it  is 
reduced.  Increase  the  strength  of  the  magnetic  field  and  again 
measure  the  current  between  P  and  P'  and  note  a  further 
reduction  of  the  current.  If  the  field  is  made  sufficiently 
strong  it  should  be  possible  to  reduce  the  current  almost  to 
zero  if  not  entirely  so. 

These  experiments  indicate  that  the  /?  rays  are  deflected  by 
the  magnetic  field,  so  that  they  strike  the  plates  BB  before  they 
escape  from  between  them,  and  consequently  do  not  reach  the 
space  between  P  and  P'.  A  weak  magnetic  field  does  not 
deflect  them  sufficiently,  and  therefore  some  of  the  rays  escape 
beyond  the  plates  BB,  but  the  stronger  the  field  the  fewer  the 
number  that  escape. 

This  deflection  of  the  /?  rays  points  to  the"  conclusion  that 
the  rays  carry  an  electric  charge.  It  is  of  importance  to  de- 
termine whether  this  charge  is  positive  or  negative.  This  may 
be  easily  determined  in  the  following  manner:  Place  a  small 
quantity  of  radium  bromide  at  the  bottom  of  a  narrow  groove 
between  two  thick  lead  plates  DD,  as  shown  in  Fig.  52,  and 
cover  it  with  the  aluminium  foil  to  cut  off  the  a  rays.  This 
groove  should  be  about  3  cm.  deep,  I  mm.  in  width  and  I  cm.  in 
length  so  as  to  obtain  a  narrow  and  sharply  defined  beam  of 
rays.  About  6  cm.  above  the  groove  place  horizontally  a  small 
photographic  plate  C,  film  side  downwards.  This  plate  should 
not  be  more  than  2  cm.  square.  Adjust  this  plate  so  that  a 
definite  marked  point  on  it  is  vertically  above  the  groove. 
Place  the  poles  of  an  electromagnet  to  cover  the  dotted  area 
NN  so  that  the  field  may  be  perpendicular  to  the  plane  of  the 
paper.  This  apparatus  should  either  be  placed  in  a  per- 
fectly dark  room  or  the  photographic  plate  and  groove  covered 


MAGNETIC   DEFLECTION    OF   RAYS  1 47 

with  an  opaque  metal  tube  to  exclude  all  light  to  prevent  the 
plate  C  becoming  fogged.  Before  applying  the  magnetic  field 
allow  the  beam  of  rays  to  act  on  the  plate  for  about  thirty 
minutes.  Then  apply  a  magnetic  field  of  about  300  or  400 
units  per  square  centimeter  in  a  known 
direction  and  allow  the  rays  to  act  f" 
again  for  about  thirty  minutes.  Re- 
move the  plate  and  develop  it  in  the 
ordinary  way,  marking,  before  removal, 


I  */ 

the  exact  position  it  occupied  relatively  '   pj]r| 

to  the  rest  of  the  apparatus.  When  de- 
veloped there  should  be  two  dark  bands 
on  the  plate,  one  of  which  is  due  to  the  i~  1 

action  of  the  rays  before  deflection  and  FlG  52> 

the  other  due  to  the  action  after  de- 
flection. The  position  of  the  latter  with  regard  to  the  former 
will  show  the  direction  in  which  the  rays  were  deflected.  This 
direction  should  be  the  same  as  the  direction  in  which  a  stream 
of  cathode  rays  would  be  deflected  under  the  action  of  the  same 
field.  This  shows  that  the  charge  carried  by  the  ft  rays  must 
be  a  negative  charge. 

93.  Magnetic  Deflection  of  a  Rays. — One  way  in  which  the 
a  rays  were  early  distinguished  from  the  ft  rays  was  that  the 
latter  were  easily  deviated  by  a  magnetic  field  while  the  former 
were  apparently  unaffected  by  such  a  field.  The  true  nature  of 
the  a  rays  was  not  known  for  some  time  after  the  nature  of  the 
ft  rays  had  been  determined.  It  was  finally  suggested  as  a 
result  of  some  indirect  experimental  evidence  that  the  a  rays 
were  positively  charged  particles  emitted  with  great  velocity. 
To  test  the  truth  of  this  suggestion  the  crucial  experiment,  of 
course,  was  to  try  to  bend  the  rays  by  a  magnetic  field.  The 
first  one  to  succeed  in  doing  this  was  Rutherford,  who  used  the 
following  method :  The  apparatus  necessary  for  the  experiment 
is  shown  in  Fig.  53.  Place  a  gold  leaf  electroscope  A  of  the 
usual  form,  of  about  10  cm.  square,  on  a  heavy  lead  plate  BB, 
in  which  an  opening  ab  is  cut.  This  opening  should  be  cov- 
ered by  a  very  thin  sheet  of  aluminium  foil  not  more  than  .0003 


148 


COMPLEXITY  OF   RADIATIONS 


cm.  in  thickness.  Below  this  is  a  set  of  twenty-five  parallel 
brass  plates  SS,  whose  planes  are  perpendicular  to  the  plane  of 
the  paper.  Make  these  plates  I  mm.  thick,  3.5  cm.  in  height 
and  i  cm.  in  width.  They  should  be  equally  spaced  apart  at 
a  distance  of  .05  cm.  This  is  done  by  cutting  grooves  equal 
distances  apart  in  two  side  plates  as  in  C  and  D,  into  which 
the  brass  plates  are  slipped.  In  the  vessel  below  these 
plates  place  a  layer  of  strongly  active  radium  bromide.  The 
rays  from  the  radium  bromide  pass  up  through  the  slits  be- 
tween the  plates  and  ionize  the  gas  in  the  electroscope.  The 
emanation  arising  from  the  radium  must  be  removed  or  else  it 
will  produce  ionization  in  the  electroscope  and  mask  the  real 
effect  to  be  observed.  This  may  be  done  by  passing  a  con- 


i\ 


H 


D 


u 
FIG.  53. 


tinuous  stream  of  dry  hydrogen  downwards  through  the  elec- 
troscope and  the  porous  aluminium  foil  and  then  through  the 
outlet  H.  The  use  of  hydrogen  instead  of  air  has  a  great 
advantage  owing  to  the  fact  that  the  a  rays  are  absorbeti-ie  a 
much  less  extent  in  hydrogen  than  in  air,  and  therefore  they 


MAGNETIC   DEFLECTION    OF   RAYS  149 

are  able  when  they  reach  the  electroscope  to  produce  greater 
effects  than  if  they  had  passed  through  air.  Hydrogen  is  pref- 
erable also  because  the  effect  of  the  ft  and  y  rays  in  the  elec- 
troscope is  less  in  hydrogen  than  in  air. 

Place  the  part  of  the  apparatus  MNOP  between  the  poles  of 
as  powerful  an  electromagnet  as  is  available  so  that  the  mag- 
netic field  is  parallel  to  the  plane  of  the  plates,  that  is,  perpen- 
dicular to  the  plane  of  the  paper  in  the  diagram.  The  strength 
of  this  field  should  be  at  least  8000  units  and  greater  if  pos- 
sible. Set  the  stream  of  hydrogen  flowing  steadily  and  per- 
form the  following  experiments:  (i)  Measure  the  ionization 
in  A,  when  no  magnetic  field  is  acting,  by  observing  the  rate  of 
discharge  of  the  gold  leaf  system  in  the  usual  manner.  (2) 
Cover  the  radium  with  a  sheet  of  aluminium  or  mica  .01  cm. 
thick  to  absorb  all  the  a  rays,  and  then  measure  the  rate  of  dis- 
charge of  the  electroscope.  The  first  observation  gives  the 
rate  of  discharge  due  to  all  three  types  of  rays,  namely,  the 
a,  ft  and  y  rays,  and  the  second  observation  gives  the  rate  of 
discharge  due  to  the  ft  and  y  rays  alone.  Therefore  the  dif- 
ference shows  the  effect  due  to  the  a  rays  alone.  The  a  rays 
will  be  found  to  produce  by  far  the  greater  amount  of  ioniza- 
tion, that  due  to  the  j3  and  y  rays  being  only  a  small  fraction 
of  the  total.  (3)  Remove  the  sheet  of  aluminium  or  mica  cov- 
ering the  radium  and  apply  the  magnetic  field  and  observe  the 
rate  of  discharge  in  the  electroscope  and  note  that  it  is  much 
less  than  in  observation  (i),  the  decrease  being  very  much 
more  than  would  be  due  to  the  cutting  off  of  the  ft  rays  by 
deflection  and  therefore  must  be  due  to  the  cutting  off  of  a 
large  proportion  of  the  a  rays  as  well.  (4)  This  may  be 
shown  by  making  another  observation.  Cover  the  radium 
again  with  aluminium  sheet  to  absorb  the  a  rays  and  apply 
the  magnetic  field  and  observe  the  rate  of  discharge  and  note 
that  it  is  slightly  less  than  in  observation  (2).  The  difference 
between  (2)  and  (4)  indicates  the  cutting  off  of  the  ft  rays 
by  the  field.  This  decrease  is  much  less  than  the  total  differ- 
ence between  (i)  and  (3),  showing  that  a  large  part  of  the 
difference  between  (i)  and  (3)  is  due  to  the  deviation  of  the 


150 


COMPLEXITY  OF  RADIATIONS 


a  rays  by  the  magnetic  field.  If  the  strength  of  field  can  be 
increased  the  difference  between  the  rates  of  discharge  in  (i) 
and  (3)  may  be  increased,  showing  that  the  stronger  the  field 
the  more  rays  are  deflected. 

These  experiments  show  that  the  a  rays  can  be  deflected  by 
a  magnetic  field,  but  it  requires  a  very  powerful  field  to  pro- 
duce appreciable  deflections,  and  it  is  for  this  reason  that  the 
a  rays  were  so  long  considered  non-deviable.  This  deviability 
of  the  rays  indicates  that  they  carry  an  electric  charge,  and 
as  in  the  case  of  the  ft  rays  it  is  of  importance  to  determine 
the  sign  of  this  charge.  This  may  be  done  in  the  following 
manner :  Arrange  another  set  of  plates  with  slits  between  them 
similar  to  those  used  in  the  last  experiments,  only  make  the 
spaces  between  the  plates  I  mm.  each  instead  of  0.5  mm.  In  a 
brass  plate  about  I  mm.  thick  cut  slits  exactly  the  same  width 
and  exactly  corresponding  to  the  slits  between  the  parallel 
plates  and  place  this  plate  over  the  vertical  plates  and  slits 
so  that  the  brass  plate  covers  a  little  over  the  half  of  the  slits 
between  the  plates  as  shown  on  an  enlarged  scale  in  Fig. 
54.  If  the  magnetic  field  is  not  quite  strong  enough  to 
deviate  all  the  a  rays,  then  if  the  devia- 
tion is  in  the  direction  from  A  to  B 
more  rays  will  escape  through  the  slits 
than  if  the  deviation  is  in  the  direction 
B  to  A.  Apply  the  magnetic  field  first 
in  one  direction  and  then  in  the  opposite 
one  and  observe  the  rate  of  discharge  in 
each  case.  This  will  show  whether  the 
FIG.  54.  ravs  are  bent  from  A  to  B  or  from  B  to 

A  when  the  field  is  applied  in  a  given 

direction.  Observe  this  carefully  and  note  that  the  a  rays  are 
bent  in  the  opposite  direction  to  that  in  which  the  ft  rays  would 
be  bent  by  the  same  field.  Since  the  /?  rays  are  negatively 
charged  the  a  rays  must  therefore  be  positively  charged. 

94.  Electrostatic  Deflection  of  the  Rays. — Since  the  mag- 
netic deflection  of  the  (3  rays  indicates  that  they  carry  a  nega- 
tive charge,  it  should  be  possible  to  deflect  them  also  by  means 


ELECTROSTATIC  DEFLECTION  OF  RAYS          151 

of  an  electrostatic  field.  This  electrostatic  deflection  may  be 
observed  by  the  photographic  method  similar  to  that  used  in 
the  case  of  the  magnetic  deflection  (§92).  Place  the  radium 
bromide  in  the  groove  formed  by  the  two  lead  plates  (Fig.  52) 
to  obtain  a  narrow  beam  of  rays.  Cut  off  the  a  rays  by  cov- 
ering the  radium  with  the  usual  sheet  of  aluminium.  Between 
the  photographic  place  C  and  the  groove  place  two  metal  plates 
parallel  to  each  other  and  to  the  plane  of  the  groove  contain- 
ing the  radium.  These  plates  should  be  about  4  cm.  high  and 
2  cm.  wide  and  I  cm.  apart. 

Before  applying  the  electric  field  allow  the  ft  rays  to  fall 
upon  the  photographic  plate  for  about  thirty  minutes  to  indi- 
cate the  undeflected  direction  of  the  rays.  Then  establish  a 
difference  of  potential  of  several  hundred  volts  between  the 
plates  and  allow  the  rays  to  fall  upon  the  photographic  plate 
for  the  same  time.  Then  develop  the  plate  and  observe  that 
the  impression  produced  by  the  deflected  beam  is  towards  the 
positive  plate.  As  in  the  case  of  the  magnetic  deflection  this 
indicates  that  the  rays  carry  a  negative  charge. 

The  a  rays  may  also  be  deflected  by  an  electrostatic  field, 
using  an  arrangement  similar  to  that  used  for  the  magnetic 
deflection  (§93),  but  in  this  case  the  parallel  plates  forming 
the  slits  (Fig.  53)  must  be  held  by  ebonite  side  pieces  C  and  D 
instead  of  metal  to  insulate  them.  Alternate  plates  should  be 
connected  together  and  a  large  difference  of  potential  estab- 
lished between  the  two  sets.  This  experiment  is  somewhat 
difficult  to  carry  out,  for  it  requires  a  very  intense  electric  field 
to  produce  an  appreciable  deflection  and  the  potential  suffi- 
cient to  produce  a  large  deviation  will  cause  a  spark  to  pass 
between  the  plates  which  are  so  close  together.  Another 
method  by  which  much  greater  effects  are  produced  will  be 
described  in  a  later  chapter  (§  no).  The  results  of  the  elec- 
trostatic deflection  of  the  a  rays  point  to  the  same  conclusion 
as  the  magnetic  deflection  does,  namely,  that  the  a  rays  carry 
a  positive  charge. 

The  y  rays  differ  from  both  the  a  and  ft  rays  in  this  matter 
of  deviation.  As  yet  no  deflection  whatever  by  either  a  mag- 


152  COMPLEXITY  OF   RADIATIONS 

netic  or  an  electric  field  has  been  observed  in  the  case  of  the 
y  rays.  They  do  not  appear  to  carry  any  charge,  as  far  as  is 
at  present  known. 

95.  Conclusions. — We  may  conclude  then  from  the  results 
of  these  experiments  that  there  are  three  definite  and  distinct 
types  of  rays  emitted  by  radio-active  substances.  The  a.  rays 
are  very  easily  absorbed  on  passing  through  solids  and  they  are 
positively  charged  particles  moving  with  a  high  velocity.  This 
high  velocity  is  indicated  by  the  fact  of  the  very  intense  mag- 
netic or  electrostatic  field  required  to  deflect  them.  The  /?  rays 
are  very  much  more  penetrating  than  the  a  rays  and  they  are 
negatively  charged  particles  emitted  with  comparatively  high 
velocity.  The  third  type,  or  y  rays,  are  extremely  penetrating, 
requiring  large  thicknesses  of  solids  to  absorb  them.  No  indi- 
cations that  they  possess  an  electric  charge  has  ever  been 
observed.  The  four  radio-active  substances  uranium,  thorium, 
radium  and  actinium  under  normal  conditions  give  out  all  three 
types  of  rays.  Polonium  however  gives  out  only  a  rays. 


CHAPTER  X. 

GENERAL  PROPERTIES   OF  RADIATIONS.      . 

96.  Methods  of  Differentiation. — To  thoroughly  investigate 
these  rays  and  to  differentiate  one  type  from  another  there  are 
different  methods  of  attack.     There  are  in  general  five  distinct 
properties,  some  of  which  we  have  already  observed,  which 
furnish  tests  that  may  be  applied  to  distinguish  the  different 
rays  from  one  another  as  follows:  (i)   Their  penetrability  or 
power  of  passing  through  different  substances;   (2)  the  ease 
with  which  they  may  be  deviated  by  a  magnetic  or  electric  field ; 
(3)  their  power  of  ionizing  gases;  (4)  their  power  of  affect- 
ing a  photographic  plate;  (5)  their  power  of  producing  phos- 
phorescence. 

These  properties  are  not  all  possessed  by  the  different  kinds 
of  rays,  but  the  presence  or  absence  of  them  or  the  degree  to 
which  they  are  present  furnish  tests  by  which  the  rays  may  be 
detected  and  differentiated.  In  general  the  rays  which  pro- 
duce the  greatest  photographic  action  produce  the  least  ioniza- 
tion.  Also  the  more  penetrating  the  rays  the  less,  efficient  are 
they  as  ionizers.  It  is  very  difficult  to  make  definite  quantita- 
tive measurements  on  the  relative  intensity  of  the  three  types 
of  rays,  whether  we  use  their  ionizing  power,  their  action  on 
a  photographic  plate  or  their  prosphorescent  action.  For  in 
each  of  these  methods  the  proportion  of  the  rays  absorbed  and 
transformed  into  the  energy  of  ionization,  or  photographic  or 
phosphorescent  energy  is  different  for  each  type  of  rays  in 
each  case,  and  only  a  portion  of  the  energy  is  transformed  into 
the  form  of  energy  used  to  detect  the  rays. 

Another  difficulty  arises  from  the  fact  that  the  three  types 
of  rays  are  usually  emitted  simultaneously  and  it  is  difficult  to 
isolate  one  type  from  the  others.  In  most  cases  however  fairly 
approximate  determinations  can  be  made. 

97.  Comparison  of  Ionization  Produced  by  a,  (3  and  y  Rays. 

'S3 


154  GENERAL   PROPERTIES   OF   RADIATIONS 

— The  a  rays  are  much  more  efficient  ionizers  than  either 
the  /?  or  y  rays.  When  all  three  types  of  rays  are  acting 
simultaneously  on  a  gas  by  far  the  greater  part  of  the  ioniza- 
tion  is  due  to  the  a  rays.  The  y  rays  are  much  less  power- 
ful ionizers  than  the  (3  rays.  A  very  approximate  idea  of 
the  relative  ionizing  powers  of  these  rays  may  be  obtained 
by  the  following  method :  In  a  lead  block  not  more  than  6  cm. 
long  cut  a  rectangular  groove  about  4  cm.  long,  0.5  cm.  wide 
and  2  cm.  deep.  In  the  bottom  of  this  place  a  thin  layer  of 
radium  bromide.  About  0.5  cm.  above  this  groove  and  parallel 
to  it  place  two  parallel  metal  plates  in  a  vertical  plane  about 
2  cm.  apart.  These  plates  should  be  about  6  cm.  high  and 
5  cm.  wide.  Pass  a  slow  steady  stream  of  air  over  the  top 
of  this  groove  to  prevent  the  emanation  from  diffusing  upward 
between  the  plates.  Place  the  lead  block  containing  the  radium 
between  the  poles  of  a  fairly  powerful  electromagnet  so  that 
the  field  is  parallel  to  the  length  of  the  groove.  Apply  a  mag- 
netic field  sufficient  to  bend  the  /?  rays  away,  so  that  they  may 
not  reach  the  space  between  the  plates,  but  not  strong  enough 
to  affect  the  a  rays.  This  will  allow  only  the  a  and  y  rays  to 
ionize  the  gas  between  the  plates.  Measure  the  saturation 
ionization  current  thus  produced  by  the  a  and  y  rays.  The 
effect  of  the  latter  is  practically  inappreciable  compared  with 
the  former.  Now  remove  the  magnetic  field  and  cover  the 
radium  with  a  sheet  of  aluminium  about  .01  cm.  thick  to  cut 
off  all  the  a  rays.  The  (3  and  y  rays  will  now  ionize  the  gas. 
Measure  the  saturation  current  under  these  conditions.  The 
greater  part  of  this  ionization  current  is  due  to  the  (3  rays. 
Now  cover  the  radium  with  a  thin  sheet  of  lead  about  2  mm. 
thick  to  cut  off  all  the  a  and  (3  rays.  Then  measure  the  ioniza- 
tion current  which  will  be  due  entirely  to  the  y  rays  and  will 
be  very  small,  so  much  so  that  the  electrometer  will  require 
to  be  very  sensitive  to  detect  it.  The  difference  between  the 
current  in  the  last  case  and  that  in  the  second  will  give  the 
current  due  to  the  ft  rays  alone,  while  the  difference  between 
the  last  and  the  first  will  give  the  current  produced  by  the  a 
rays  alone.  The  relative  amount  of  ionization  produced  by  the 


RELATIVE    IONIZATION    BY    RAYS  155 

three  types  of  rays  under  the  same  conditions  may  thus  be  com- 
pared. It  will  be  seen  that  the  a  rays  produce  several  thou- 
sand times  as  much  ionization  as  the  y  rays,  while  the  /Trays 
produce  ionization  of  the  order  of  about  one  hundred  times 
that  of  the  y  rays. 

If  similar  experiments  are  made,  using  the  other  radio-active 
substances  such  as  thorium  or  uranium,  it  will  be  found  that 
the  relative  ionizing  power  of  the  three  types  of  rays  are  in 
the  same  order  as  in  the  case  of  radium.  It  will  be  observed 
also  that  the  (3  rays  emitted  by  either  thorium  or  uranium  are 
very  weak  ionizers. 

By  modifying  the  apparatus  slightly  the  relative  ionization 
of  the  different  types  of  rays  may  be  compared  in  different 
gases.  An  arrangement  suitable  for 
this  is  shown  in  Fig.  55.  A  is  the  lead 
block  with  the  groove  B  cut  in  it  as 
before.  The  plates  P  and  P  between 
which  the  ionization  is  to  be  measured 
are  contained  in  a  brass  tube  MN  about 
10  cm.  high  and  6  cm.  diameter.  These 
plates  are  supported  by  stout  rods  pass- 
ing out  through  ebonite  insulators  as 
shown.  This  enclosing  tube  should  be 

made  to  fit  closely  on  the  flat  surface  of  the  lead  block  and  the 
joints  made  air-tight  by  waxing.  For  these  experiments  use 
a  sample  of  an  active  uranium  salt  as  uranium  emits  no  emana- 
tion while  the  other  active  substances  do. 

Place  the  uranium  in  the  groove  and  then  carefully  wax 
down  the  tube  MN  and  place  the  lead  block  between  the  poles 
of  the  electromagnet  as  before.  Start  with  the  vessel  filled 
with  air  at  atmospheric  pressure,  and,  after  bending  the  /?  rays 
out  of  the  way  by  the  magnetic  field,  measure  the  ionization 
produced  by  the  a  rays.  Exhaust  the  vessel  and  fill  with  other 
gases  in  turn  and  measure  the  ionization  current  due  to  the 
a  rays  in  each  case.  Now  remove  the  magnetic  field  and  also 
remove  the  vessel  MN  and  cover  the  groove  with  the  alumin- 
ium sheet  to  cut  off  the  a  rays  and  replace  the  vessel  and  rewax 


156  GENERAL   PROPERTIES   OF   RADIATIONS 

the  joint.  Measure  now  the  ionization  produced  by  the  ft  rays 
in  the  same  gases  as  were  used  in  the  case  of  the  a  rays.  Make 
a  careful  comparison  in  each  case  and  it  will  be  found  that  the 
numbers  representing  the  relative  saturation  currents  pro- 
duced in  the  different  gases  by  the  a  rays  are  not  in  exactly  the 
same  ratio  as  those  representing  the  currents  produced  by  the 
ft  rays,  although  they  follow  in  the  same  general  order. 

If  the  uranium  is  covered  by  a  thin  sheet  of  lead  to  cut  off 
both  the  a  and  /3  rays  it  will  be  found  that  the  ionization 
produced  by  the  y  rays  is  very  small  in  the  various  gases,  and 
unless  the  electrometer  is  a  very  sensitive  one  they  will  have 
to  be  measured  by  means  of  an  electroscope. 

98.  Photographic  Action  of  the  Rays. — Although  the  a  rays 
are  much  more  active  as  ionizers  than  the  ft  or  y  rays,  when 
we  come  to  study  the  action  of  the  different  types  of  rays  on 
a  photographic  plate  it  is  found  that  the  ft  rays  are  much 
more  active  photographically  than  either  the  a  or  y  rays. 
The  y  rays  are  also  much  less  active  photographically  than  even 
the  a  rays,  and  in  the  case  of  the  very  weak  y  radiation  from 
uranium  and  thorium  practically  no  photographic  effect  has 
been  found.  In  making  experimental  determinations  on  these 
radiations  this  difference  between  the  ionizing  and  photo- 
graphic properties  of  the  a  and  ft  rays  especially  must  be  care- 
fully, taken  into  account,  for  otherwise  contradictory  results 
are  very  apt  to  be  obtained.  Take  as  an  instance  a  case  when 
only  one  kind  of  rays  are  being  used,  such  as  the  a  rays,  for 
instance.  If  the  ionization,  or  electrical,  method  of  measure- 
ment is  used  quite  a  large  effect  may  be  observed  from  the 
active  body,  while  if  the  photographic  method  of  measurement 
is  employed  the  effect  may  be  very  small  or  may  even  not  be 
detected  at  all.  If  however  the  ft  rays  emitted  by  the  active 
body  had  been  used  just  the  reverse  would  have  been  observed, 
for  the  photographic  action  would  be  comparatively  strong, 
while  the  ionization  effect  would  be  comparatively  weak. 

It  is  somewhat  difficult  to  accurately  compare  the  relative 
photographic  action  produced  by  the  different  types  of  rays, 
for  in  many  cases  the  photographic  plate  must  be  wrapped  in 


PHOTOGRAPHIC   ACTION  157 

black  paper  to  cut  off  the  phosphorescent  light  which  is  also 
given  out  by  some  of  the  radio-active  bodies,  and  this  paper 
will  absorb  the  a  rays  much  more  than  it  will  the  ft  or  y  rays. 
Then  in  the  case  of  uranium  or  thorium  where  the  radiations 
are  comparatively  weak  it  requires  generally  about  a  day's 
exposure  to  produce  an  appreciable  effect  on  the  plate,  and 
during  this  time  other  effects  from  extraneous  sources  are  apt 
to  occur  unless  very  special  precautions  are  taken.  A  very 
approximate  comparison  may  be  made  by  the  following  method : 
Place  a  specimen  of  radium  bromide  in  the  groove  of  a  lead 
block  similar  to  the  one  shown  in  Fig.  55.  Wrap  a  photo- 
graphic plate  about  7  cm.  square  in  a. sheet  of  black  paper  to 
protect  it  from  any  light  and  place  it  horizontally,  film  down- 
wards, about  3  or  4  cm.  above  the  lead  block.  The  experiment 
should  be  done  in  a  dark  room.  Bend  the  ft  rays  out  of  the 
way  in  the  usual  manner  and  allow  the  a  rays  to  fall  upon  the 
plate  for  about  an  hour  or  more.  Now  cover  the  radium  with 
the  usual  absorbing  sheet  of  aluminium  to  absorb  the  a  rays 
and  remove  the  magnetic  field.  Move  the  photographic  plate 
so  that  another  portion  of  it  is  above  the  groove  and  exposed 
to  the  rays.  Allow  the  ft  rays  to  fall  upon  the  plate  for  the 
same  length  of  time  as  in  the  first  instance.  Again  cover  the 
radium  with  the  usual  absorbing  lead  sheet  to  absorb  the  a 
and  ft  rays  and  shift  the  plate  again  so  as  to  expose  another 
portion  to  the  y  rays.  Allow  the  y  rays  alone  to  act  for  the 
same  time  as  in  the  other  cases.  Remove  the  plate  and  develop 
it  and  compare  the  three  impressions  made  by  the  three  types 
of  rays.  This  experiment  might  be  repeated  for  shorter  times 
as  well  as  longer  times  of  exposure. 

The  experiments  should  be  repeated  using  samples  of  uran- 
ium and  of  thorium.  The  time  of  exposure  with  these  sub- 
stances will  require  to  be  very  much  longer  to  obtain  any 
appreciable  effect.  It  will  require  about  a  day's  exposure  in 
each  case.  It  will  be  found  also  that  the  y  rays  from  uranium 
and  thorium  produce  practically  no  effect. 

99.  Phosphorescent  Action  of  the  Rays. — The  rays  emitted 
by  the  different  radio-active  bodies  cause  certain  substances 


158  GENERAL   PROPERTIES   OF   RADIATIONS 

to  phosphoresce  when  the  rays  fall  upon  them.  This  effect 
may  be  observed  in  the  case  of  quite  a  large  number  of  sub- 
stances such  as  crystals  of  zinc  sulphide,  platino-cyanide  of 
barium,  diamond,  lithium,  willemite,  kunzite,  etc.  The  various 
substances  which  show -this  effect  are  not  equally  affected  by 
the  three  types  of  rays.  For  instance,  zinc  sulphide  is  espe- 
cially sensitive  to  the  action  of  the  a  rays,  while  the  platino- 
cyanides  of  barium  or  lithium  show  the  effect  of  the  y  rays  to 
a  marked  degree.  These  phosphorescent  effects  are  in  some 
cases  very  brilliant,  and  they  serve  as  a  very  convenient  means 
of  detecting  and  observing  these  radiations. 

In  the  case  of  some  of  the  phosphorescent  bodies  there  is  a 
marked  phenomenon  shown  by  the  a  rays  which  is  not  shown 
by  the  ft  or  y  rays.  When  the  a  rays  fall  upon  zinc  sulphide, 
for  instance,  it  is  brilliantly  illuminated,  and  if  it  be  examined 
with  a  magnifying  glass  the  illumination  is  found  not  to  be  uni- 
formly distributed,  but  to  consist  of  bright  scintillating  points 
as  though  the  zinc  sulphide  were  being  bombarded  and  a  bright 
scintillation  resulted  from  each  impact.  The  illumination  pro- 
duced by  the  (3  or  y  rays  differs  from  this  in  being  uniform 
and  continuous,  and  does  not  show  these  peculiar  scintillations. 
This  scintillating  action  is  shown  by  the  a  rays  emitted  by 
practically  all  the  radio-active  bodies.  The  phenomenon  is 
most  marked  with  zinc  sulphide,  but  it  may  also  be  observed 
with  willemite  and  the  platino-cyanide  of  potassium. 

On  a  sheet  of  thick  white  paper  or  very  thin  white  cardboard 
dust  a  uniform  layer  of  finely  powdered  crystalline  zinc  sul- 
phide. The  cardboard  should  first  be  coated  with  a  thin  coat- 
ing of  paste  to  cause  the  sulphide  crystals  to  adhere.  In  a  dark 
room  place  the  screen  horizontally  about  2  cm.  above  a  speci- 
men of  radium.  The  screen  should  appear  brilliantly  illumi- 
nated. Examine  this  illumination  carefully  with  a  magnifying 
glass  and  observe  the  bright  flashes  or  scintillations.  Cover  the 
radium  with  the  usual  absorbing  sheet  of  aluminium  to  cut  off 
all  the  a  rays.  Note  that  the  illumination  diminishes  in  in- 
tensity and  not  only  that  but  no  scintillations  are  visible,  show- 
ing that  a  large  part  of  the  total  illumination  is  due  to  the 


ABSORPTION    OF   RAYS  159 

a  rays"  and  also  that  the  scintillating  action  must  be  due  to  the 
action  of  the  a  rays. 

If  the  ft  rays  be  also  cut  off  by  an  absorbing  metal  sheet  the 
phosphorescence  produced  by  the  y  rays  alone  may  be  ob- 
served. This  action  will  be  found  weaker  than  that  of  either 
the  a  or  /3  rays.  A  screen  made  of  crystalline  platino-cyanide 
of  barium  or  lithium  is  much  more  suitable  for  showing  the 
phosphorescent  action  of  the  y  rays  than  the  other  phosphores- 
cent substances. 

100.  Complexity  of  a  and  ft  Rays  from  Radium. — When  a 
narrow  beam  of  (3  rays  from  radium  is  allowed  to  fall  upon  a 
photographic  plate  or  a  phosphorescent  screen  a  narrow  dark 
or  bright  band  is  produced.     If  a  magnetic  field  be  applied  to 
the  beam  of  rays  it  will  not  only  be  deviated  as  shown  by  the 
movement  of  the  band  on  the  photographic  plate  or  phosphor- 
escent screen,  but  the  band  will  also  be  increased  in  width, 
showing  that  some  of  the  rays  in  the  beam  must  have  been 
bent  more  than  others.     This  indicates  that  the  £  rays  from 
radium  are  not  perfectly  homogeneous.     Some  of  them  are 
projected  with  greater  velocity  than  others  and  those  with  the 
higher  velocity  will  of  course  be  deviated  less  by  a  given  mag- 
netic field.     The  (3  rays  emitted  by  uranium  do  not  show  this 
broadening,  indicating  that  the  beam  of  (3  rays  from  uranium 
is  homogeneous  in  character,  the  rays  all  being  emitted  with 
the  same  velocity. 

The  a  rays  from  radium  also  show  a  similar  want  of  homo- 
geneity. It  is  not  so  easily  observed  in  the  case  of  a  rays  on 
account  of  the  difficulty  in  bending  them.  Careful  experi- 
ments show  that  the  a  rays  emitted  by  radium  are  not  all  pro- 
jected with  the  same' velocity. 

101.  Absorption  of  the  Rays  by  Solids. — We  have   seen 
(§90)  that  one  of  the  most  marked  distinguishing  character- 
istics of  the  three  types  of  rays  is  their  different  powers  of 
penetrating  solid  bodies.     Some  further  experiments  on  this 
question  may  prove  of  value.     Uranium  will  be  found  a  more 
convenient  source  of  a  and  ft  rays  for  the  study  of  this  ques- 
tion than  radium  on  account  of  the  complexity  of  both  the 


l6o  GENERAL   PROPERTIES    OF   RADIATIONS 

a  and  ft  rays  from  radium,  but  radium  is  better  as  a  source  of 
y  rays  as  it  gives  out  a  much  more  intense  y  radiation  than 
uranium. 

a  Rays. — The  form  of  apparatus  shown  in  Fig.  50  will  be 
found  convenient  for  the  examination  of  the  absorption  of  the 
a  and  ft  rays.  If  a  very  thin  layer  of  uranium  be  used  prac- 
tically all  the  ionization  produced  is  due  to  the  a  rays,  only 
about  one  or  two  per  cent,  of  the  total  ionization  being  due  to 
the  ft  and  y  rays.  Place  a  very  thin  layer  of  uranium  oxide  of 
about  25  sq.  cm.  in  area  on  the  plate  C  (Fig.  50)  and  adjust  the 
distance  between  the  plates  to  about  2  cm.  Measure  the  satu- 
ration ionization  current  between  the  plates.  Place  a  thin  sheet 
of  aluminium  foil,  not  more  than  .0003  cm.  thick,  over  the 
uranium  and  measure  the  current.  Repeat  this,  adding  a  sheet 
at  a  time,  till  all  the  a  rays  are  completely  cut  off.  Repeat 
this  using  very  thin  sheets  of  tissue  paper. 

The  a  rays  of  radium  may  be  separated  from  the  ft  rays 
temporarily  by  dissolving  a  little  radium  chloride  in  water  and 
then  evaporating  it  on  a  metal  plate.  The  radium  chloride  left 
on  the  plate  is  thus  rendered  nearly  free  from  ft  rays  for  a 
short  time.  Using  such  a  deposit  as  a  source  of  radiation, 
repeat  the  experiments  with  the  aluminium  foil. 

If  /0  is  the  intensity  of  the  rays  before  passing  through  any 
absorbing  material  and  /  the  intensity  after  passing  through  a 
thickness  x  of  the  absorbing  substance,  it  will  be  found  that 
the  relation  between  70  and  /  obeys  approximately  the  same 
law  as  found  for  Rontgen  rays  (§61)  under  similar  conditions, 
namely,  7//0  =  e~Xa?,  where  A  is  the  coefficient  of  absorption. 

ft  Rays. — To  investigate  the  absorption  of  the  ft  rays  use  a 
comparatively  thick  layer  of  uranium  oxide  several  millimeters 
thick.  Cover  this  with  a  sufficient  thickness  of  aluminium  foil 
to  cut  off  all  the  a  rays.  Perform  a  set  of  experiments  similar 
to  those  made  with  the  a  rays.  In  this  instance,  however,  since 
the  ft  rays  are  so  much  more  penetrating,  a  great  thickness  of 
absorbing  material  may  be  used  and  therefore  a  much  greater 
variety  of  substances  may  be  tested.  Test  as  before  the  rela- 
tion between  the  absorption  and  the  thickness  of  the  absorbing 


ABSORPTION   OF   RAYS  l6l 

material  and  note  that  the  absorption  law  given  above  is  ap- 
proximately adhered  to  in  the  case  of  the  ft  rays  as  well  as  the 
a  rays.  Compare  also  the  absorbing  power  of  equal  thick- 
nesses of  different  materials  and  observe  that  the  absorption 
increases  with  the  density,  but  they  are  not  proportional  to  one 
another. 

y  Rays. — The  absorption  of  the  y  rays  may  be  most  success- 
fully examined  by  using  a  layer  of  radium  a  millimeter  or  so 
in  thickness  as  the  source  of  rays,  and  an  electroscope  for 
measuring  the  intensity  of  the  rays. 

Place  a  layer  of  radium  bromide  in  a  shallow  receptacle  of 
not  more  than  i  or  2  mm.  depth  and  10  or  12  sq.  cm.  area. 
Cover  it  with  a  thin  sheet  of  mica  about  .01  cm.  in  thickness 
and  carefully  seal  down  the  edges  with  wax  to  prevent  the 
emanation  from  escaping.  Place  an  electroscope  of  the  usual 
form  (Fig.  14)  on  a  lead  plate  in  which  is  cut  an  opening  about 
6  cm.  square.  About  2  or  3  cm.  below  this  opening  place  the 
enclosed  radium  and  cover  it  with  a  sheet  of  lead  about  2  or 
3  mm.  thick  so  as  to  completely  cut  off  the  ft  rays.  Measure 
the  intensity  of  the  y  rays  by  the  rate  of  discharge  of  the  elec- 
troscope. Then  cover  the  radium  with  gradually  increasing 
thicknesses  of  absorbing  materials  and  note  the  diminution  in 
intensity  with  increase  of  thickness  for  each  substance.  Test 
this  by  the  absorption  equation  given  above.  Compare  also  the 
absorbing  powers  of  different  substances  for  y  rays.  It  will 
be  found  that  the  thicknesses  of  absorbing  materials  required 
in  these  experiments  to  produce  appreciable  absorption  are 
very  much  greater  than  in  the  experiments  with  a  and  ft  rays. 
Quite  appreciable  effects  will  be  produced  by  the  y  rays  even 
after  they  have  passed  through  several  centimeters  of  such  a 
dense  substance  as  lead.  In  all  these  experiments  on  absorp- 
tion it  will  be  found  instructive  to  plot  curves  showing  the 
relation  between  the  intensity  of  the  rays  after  passing  through 
the  absorbing  material  and  the  thickness  of  the  absorbing 
substance. 

102.  Effect  of  Varying  the  Thickness  of  a  Layer  of  Radio- 
active Material. — In  some  of  our  experiments  we  have  used 

12 


1 62  GENERAL   PROPERTIES   OF   RADIATIONS 

as  our  source  of  radiation  a  thin  layer  of  radio-active  material 
and  in  others  a  thick  layer.  The  reason  for  this  will  now  be 
apparent  after  studying  the  absorptive  power  of  solid  bodies 
for  these  radiations.  The  radiations  are  emitted  from  all  parts 
of  the  radio-active  material,  not  only  at  the  surface  but  through- 
out the  body  of  the  substance.  If  there  were  no  such  thing 
as  absorption  then  the  rays  from  all  parts  of  the  material 
would  emerge  into  the  air  with  equal  intensity  and  produce 
their  effects  and  any  increase  in  the  quantity  of  material  would 
produce  a  proportional  increase  in  the  amount  of  radiation 
emitted.  But  since  absorption  does  take  place,  when  the  layer 
of  material  is  thick  then  the  rays  which  are  given  off  from  the 
lower  layers  have  to  pass  through  the  upper  layer  before  emerg- 
ing into  the  air  and  therefore  suffer  absorption  in  passing 
through  the  solid  material,  whereas  if  the  layer  is  very  thin 
practically  no  absorption  takes  place,  as  the  distance  traversed 
is  so  very  short.  On  account  of  the  difference  in  the  penetrat- 
ing powers  of  the  three  types  of  rays  the  effect  of  altering  the 
thickness  of  the  radiating  material  is  different  in  the  three 
cases.  Consider  the  effect  on  each  type  of  ray  separately. 

Effect  on  a  Rays. — The  a  rays  given  off  from  a  very  thin 
layer  suffer  no  absorption  as  they  are  all  practically  given  off 
from  the  surface  and  the  full  effect  of  the  rays  is  observed  in 
the  air  above  the  material.  If  the  layer  of  material  is  appre- 
ciably increased  the  amount  of  radiation  emitted  is  increased  in 
the  same  proportion,  but  the  rays  emitted  from  the  lower  layer 
have  to  pass  through  the  increased  thickness  of  material  and  on 
account  of  their  weak  penetration  suffer  absorption  and  only  a 
portion  of  them  finally  emerge  into  the  air.  Therefore  the  in- 
crease in  the  emergent  radiation  is  not  proportional  to  the 
increase  in  thickness.  The  thicker  the  layer  the  more  are  the 
a  rays  from  below  absorbed,  until  finally  a  thickness  is  reached 
which  is  sufficient  to  completely  absorb  all  the  a  rays  which 
come  from  a  depth  below  this  thickness.  Consequently  when 
this  condition  is  reached  any  further  increase  in  the  thickness 
of  the  material  will  produce  no  further  increase  in  the  emer- 
gent a  radiation.  Starting,  therefore,  with  a  very  thin  layer  of 


ABSORPTION    OF   RAYS  163 

material  the  ionization  will  increase  with  increase  in  thickness 
until  a  maximum  is  reached  at  the  point  where  the  thickness 
is  sufficient  to  absorb  all  the  a  rays  coming  from  below  this 
depth  and  the  maximum  will  remain  constant  for  any  increase 
in  material. 

/?  Rays. — The  effect  is  very  much  less  marked  in  the  case  of 
the  (3  rays.  The  latter  are  so  much  more  penetrating  than  the 
a  rays  that  they  will  pass  through  a  much  greater  thickness  of 
radio-active  material  without  suffering  much  absorption.  If 
the  ionization  produced  by  the  (3  rays  from  a  thin  layer  is 
measured  and  the  thickness  of  material  gradually  increased  the 
ionization  will  increase  much  more  nearly  in  proportion  to  the 
amount  of  material,  for  the  ft  rays  from  the  lower  layers  are 
absorbed  to  such  a  small  extent  that  covering  the  lower  layer 
with  more  radio-active  material  does  not  produce  much  absorp- 
tion. Finally,  of  course,  a  thickness  will  be  reached  which 
will  absorb  all  the  /?  rays  emitted  from  below  this  thickness 
but  the  quantity  of  material  will  be  very  much  greater  than 
that  required  to  absorb  the  a  rays. 

y  Rays. — The  y  rays  are  so  extremely  penetrating  that  they 
will  pass  through  a  very  large  quantity  of  radio-active  material 
before  being  absorbed  to  any  appreciable  extent.  The  ioniza- 
tion produced  by  the  y  rays  alone  will  therefore  be  practically 
proportional  to  the  quantity  of  radio-active  material  used.  To 
obtain  an  intense  y  radiation  a  large  quantity  of  material  may 
be  employed. 

Experimental  Tests. — The  results  given  above  may  be  very 
easily  observed  experimentally.  Use  the  apparatus  shown  in 
Fig.  50  and  as  a  receptacle  for  holding  the  radio-active  material 
use  the  one  described  in  §  85,  so  as  to  use  the  same  surface 
area  in  all  cases.  The  detachable  plate  with  the  hole  in  it 
should  be  about  i  cm.  thick  to  give  sufficient  depth  and  the 
hole  in  the  plate  should  be  about  5  cm.  square.  Place  as  thin  a 
layer  as  possible  of  uranium  oxide  on  the  bottom  of  the  re- 
ceptacle. This  may  be  done  by  'sifting  the  oxide  through  a 
thin  wire  gauze  as  uniformly  as  possible.  Measure  the  cur- 
rent between  the  plates,  Weigh  the  plate  before  the  oxide  is 


164  GENERAL    PROPERTIES    OF    RADIATIONS 

placed  on  it  and  also  afterwards  to  obtain  the  weight  of  oxide. 
Increase  the  thickness  of  oxide  slightly  and  again  measure  the 
current  and  weigh  the  oxide.  Continue  this  until  the  current 
approaches  a  maximum.  If  the  material  is  uniformly  sifted 
over  the  area  the  thickness  may  be  taken  as  proportional  to  the 
weight  in  each  case.  Plot  a  curve  showing  the  relation  be- 
tween the  thickness  and  the  current  produced.  With  the 
thicker  layers  the  p  rays  become  more  prominent  and  part  of 
the  current  is  due  to  them. 

Start  again  with  a  thin  layer  and  cover  it  with  a  sheet  of 
aluminium  thick  enough  to  absorb  all  the  a  rays.  Repeat  the 
above  experiments  and  observe  the  relation  between  the  thick- 
ness of  material  and  the  ionization  produced  by  the  ft  rays. 

The  quantity  of  a  rays  emitted  by  uranium  oxide  is  very 
small  and  besides  that  the  y  rays  are  very  weak  ionizers.  Con- 
sequently the  y  radiation  from  a  very  thin  layer  of  uranium  is 
practically  inappreciable  and  considerable  thickness  is  required 
to  produce  much  effect.  Start  with  a  layer  a  couple  of  milli- 
meters thick  and  cover  it  with  a  lead  plate  about  two  millime- 
ters in  thickness  to  absorb  all  the  a  and  (3  rays.  Repeat  the 
above  experiments  and  compare  the  ionization  produced  by 
different  thicknesses  of  material.  To  measure  the  ionization 
use  the  usual  form  of  electroscope. 

Test  also  the  y  radiation  from  specimens  of  thorium  and  of 
radium.  In  these  tests  the  emanation  will  not  interfere,  as  the 
thorium  or  radium  may  for  this  purpose  be  covered  with  a 
sheet  of  mica  and  sealed  up  so  as  to  be  air-tight.  Note  that  it 
requires  a  considerable  thickness  of  thorium  to  produce  ap- 
preciable ionization  by  the  y  rays,  but  that  a  much  thinner 
layer  of  radium  will  produce  as  great  if  not  greater  effects. 

103.  Absorption  of  Rays  by  Gases. — The  rays  from  radio- 
active substances  suffer  absorption  in  their  passage  through 
gases  as  well  as  through  solids,  but  of -course  to  a  much  less 
extent.  The  y  rays  are  so  penetrating  that  the  absorption  of 
them  by  gases  is  extremely  small.  The  ft  rays  will  pass  through 
a  considerable  distance  without  showing  much  diminution  in 
intensity  but  the  distance  is  much  less  than  in  the  case  of  y  rays. 


ABSORPTION    OF   RAYS 


i6s 


The  a  rays,  on  account  of  their  low  penetrating  power,  are 
comparatively  easily  absorbed  by  gases.  This  absorption  of 
the  a  rays  by  gases  may  be  easily  observed  by  means  of  the 
apparatus  shown  in  Fig.  56. 

AB  is  a  brass  plate  12  cm.  square  supported  in  a  fixed  posi- 
tion by  a  brass  rod  R  passing  out  through  an  ebonite  plug. 
Parallel  to  AB  and  attached  to  it  by  ebonite  rods  is  the  plate 
CD  of  the  same  size  in  which  is  cut  an  opening  about  8  cm. 
square.  This  opening  is  covered  with  extremely  thin  alumin- 
ium foil  or  very  thin  wire  gauze.  The  distance  between  these 
plates  should  be  about  1.5  cm.  AB  is  connected  to  the  electrom- 


M 
— 

E 

1 

Miy| 

EARTH 

a     Illl    fill 

A     T, 

I 

a 

« 

P 

75 

c 

p 

a    |l|l  |l|l 

EARTH 
A/ 

7 

I 


FIG.  56. 

eter  and  CD  to  a  battery  in  the  usual  manner.  A  plate  P  about 
14  cm.  square  is  supported  by  a  rod  S,  which  passes  through 
a  closely  fitting  tube  and  nut  attached  to  the  base  plate  HK. 
By  means  of  this  nut  P  may  be  raised  or  lowered  parallel  to 
itself  so  the  distance  between  P  and  CD  may  be  altered  as 
desired.  A  piece  of  rubber  tubing  fits  tightly  over  the  lower 


l66  GENERAL    PROPERTIES   OF   RADIATIONS 

part  of  this  nut  and  the  rod  S  so  as  to  make  the  joint  air- 
tight. The  whole  system  of  plates  is  enclosed  in  a  metal  box 
MTV  which  fits  on  to  the  base  plate  HK.  The  joints  may  all 
be  made  air-tight  by  waxing.  There  should  be  a  couple  of 
windows  in  the  sides  of  the  vessel  to  observe  the  interior.  The 
radio-active  material  is  placed  in  a  uniform  layer  on  the  plate 
P  covering  an  area  of  12  cm.  square.  Connect  the  plate  P  and 
the  enclosing  vessel  to  earth.  The  rays  from  the  radio-active 
material  on  P  ionize  the  fixed  volume  of  gas  between  the  plates 
AB  and  CD  between  which  the  current  is  measured.  Since 
the  rays  are  absorbed  in  passing  through  the  gas  between  P  and 
CD  we  should  expect  the  current  between  AB  and  CD  to 
diminish  as  the  distance  between  P  and  CD  is  increased.  Place 
a  thin  layer  of  uranium  oxide  on  P  so  that  practically  all  the 
ionization  is  produced  by  the  a  rays  and  the  /?  rays  may  be 
neglected.  Start  with  P  a  couple  of  millimeters  or  so  from  CD 
and  measure  the  current,  then  gradually  lower  the  plate  P  by 
small  measured  intervals  and  measure  the  current  at  each  dis- 
tance and  observe  how  the  current  diminishes  with  increase  of 
distance.  Plot  a  curve  showing  the  relation  between  current 
and  distance  between  P  and  CD.  It  will  be  seen  that  as  the 
distance  increases  in  arithmetical  progression  the  current 
diminishes  approximately  in  geometrical  progression. 

As  was  shown  in  §  101  for  solids  if  J0  is  the  intensity  of 
the  a  rays  at  the  surface  of  the  radio-active  material  and  7 
the  intensity  at  a  distance  x  from  it  then  I  =  70€~Xa?,  where  A  is 
the  coefficient  of  absorption  for  the  gas.  Let  x  be  the  dis- 
tance between  P  and  CD  and  /  the  distance  between  CD  and 
AB.  The  rays  are  also  absorbed  in  passing  through  the  foil 
F  by  a  small  constant  fraction.  Let  the  fraction  which 
emerges  after  passing  through  the  foil  be  denoted  by  K.  Then 
the  intensity  of  the  rays  at  the  upper  surface  of  F  is  equal  to 
KI0e-*x  and  at  the  lower  surface  of  AB  it  is  KIQc^(x+l\  Then 
since  the  amount  of  ionization  produced  is  proportional  to  the 
intensity  of  the  rays  the  number  of  ions  produced  between 
the  plates  is  proportional  to  KI0c^x  —  A!70e~X(a?+Z),  that  is  to 
KI0(i—  e-x')e~Xa?-  Therefore  since  KI0(i—  e~xz)  is  constant 


ABSORPTION    OF    RAYS  167 

for  given  conditions  the  saturation  current  between  the  plates 
is  proportional  to  €~Xa?,  that  is  it  decreases  according  to  an  ex- 
ponential law  with  the  increase  of  distance  from  the  source  as 
was  observed  by  experiment. 

Fill  the  vessel  with  other  gases  in  turn  and  repeat  the  experi- 
ments and  plot  the  corresponding  curves,  using  distances 
between  CD  and  P  as  abscissae  and  currents  as  ordinates. 

Since  the  current  C  is  proportional  to  €~x*  and  also  propor- 
tional to  the  deflection  d  of  the  electrometer 'needle,  then  d  is 
proportional  to  c~Xar.  By  observing  dl  and  d2  for  two  known 
distances  x^  and  xz  in  any  gas  and  supplying  these  values  in 
the  equation  dJdz^=ckx^./cKx^  the  value  of  the  coefficient  of 
absorption  A  may  be  determined  for  this  gas.  Calculate  from 
the  observations  the  value  of  A  in  each  gas. 

By  more  elaborate  and  careful  measurements  it  has  been 
found  that  the  relative  ionization  in  gases  is  proportional  to 
the  relative  absorption.  The  ionization  produced  by  the 
rapidly  moving  a  particles  is  due  to  their  collisions  with  the 
molecules  of  the  gas.  The  more  collisions  that  take  place  the 
more  ions  will  be  produced.  It  requires  energy  to  produce 
these  ions,  and  the  energy  is  derived  from  the  kinetic  energy  of 
the  a  particles.  The  energy  of  the  a  particles  is  thus  gradu- 
ally reduced  in  their  passage  through  the  gases,  and  if  their 
energy  is  reduced  below  a  certain  amount  they  do  not  possess 
sufficient  energy  to  produce  ions  and  therefore  lose  their  power 
of  manifesting  their  presence.  The  energy  of  the  rays  is  thus 
absorbed  and  there  is  therefore  a  direct  relation  between  the 
amount  of  absorption  and  the  amount  of  ionization  in  any 
given  gas. 

This  method  is  not  a  convenient  one  to  measure  the  ab- 
sorption of  either  the  ft  or  y  rays  by  gases,  for  on  account  of 
their  much  greater  penetrating  power  it  would  require  a  great 
distance  between  the  plates  P  and  CD  to  produce  an  appre- 
ciable absorption  which  could  be  measured  with  accuracy.  If 
greater  distances  were  used  to  produce  sufficient  absorption  to 
be  measured  the  rays  would  spread  out  to  such  an  extent  that 
the  intensity  of  radiation  could  not  be  considered  constant  over 
a  plane  parallel  to  the  plate  P  at  a  great  distance  from  P. 


1 68  GENERAL   PROPERTIES   OF   RADIATIONS 

Using  the  same  apparatus  repeat  these  experiments  for  each 
of  the  gases  at  different  pressures  below  an  atmosphere.  Ob- 
serve that  for  any  given  gas  the  absorption  is  very  approxi- 
mately proportional  to  the  pressure  of  the  gas  as  would  be 
expected. 

104.  Effect  of  Pressure  on  lonization.— The  rate  at  which 
ions  are  produced  by  the  radiations  from  radio-active  bodies 
depends  upon  the  pressure  of  the  gas.  In  the  study  of 
Rontgen  rays  (§62)  it  was  seen  that  the  saturation  current  be- 
tween two  parallel  plates  was  proportional  to  the  pressure.  In 
this  instance  the  ionization  between  the  plates  was  practically 
uniform.  The  same  is  true  for  the  ionization  produced  by  the 
rays  from  radio-active  substances  if  the  ionization  between  the 
plates  is  uniform.  If  a  radio-active  body  giving  out  a  rays 
be  placed  on  the  plate  C  (Fig.  50)  which  is  placed  at  a  distance 
of  4  cm.  from  D  and  the  current  measured  in  air  at  atmospheric 
pressure  and  also  at  gradually  decreasing  pressure  below  an 
atmosphere  the  current  will  be  found  to  decrease  as  the  pres- 
sure decreases,  but  at  first  the  decrease  of  current  will  be  less 
rapid  than  the  decrease  of  pressure  until  a  pressure  of  about 
half  an  atmosphere  is  reached.  For  pressures  below  this  the 
current  will  decrease  in  exact  proportion  to  the  pressure.  If 
the  vessel  be  filled  with  carbon  dioxide  in  place  of  air  the 
want  of  proportionality  will  continue  till  the  pressure  is  in  the 
neighborhood  of  about  one  third  of  an  atmosphere  and  below 
that  the  current  will  be  proportional  to  the  pressure.  These 
results  appear  at  first  sight  to  be  contrary  to  the  statement  that 
the  ionization  is  proportional  to  the  pressure,  but  in  reality  they 
follow  as  a  natural  consequence  of  the  absorption  of  the  rays 
by  the  gas.  Since  the  a  rays  which  produce  the  greater  part 
of  the  ionization  are  so  very  easily  absorbed  by  the  gas  the  in- 
tensity of  the  rays  is  greatest  at  the  surface  of  the  plate  C  and 
gradually  diminishes  towards  D.  The  ionization  is  therefore 
not  uniform  between  the  plates,  as  more  ions  are  produced 
near  C  than  near  D.  As  the  pressure  is  decreased  the  absorp- 
tion is  less  and  at  first  the  rays  are  able  to  penetrate  with 
greater  intensity  into  the  region  nearer  D  and  are  therefore 


IONIZATION    AND    PRESSURE 


169 


able  to  produce  more  total  ionization.  The  increased  intensity 
therefore  partly  counterbalances  the  decreased  pressure.  This 
condition  of  affairs  continues  until  such  a  pressure  is  reached 
at  which  the  rays  are  able 
to  penetrate  to  D  with 
practically  their  full  inten- 
sity, and  the  ionization  be- 
tween the  plates  is  uni- 
form over  the  whole  space 
and  remains  uniform  for 
any  further  decrease  of 
pressure.  From  this  stage 
then  the  ionization  is  pro- 
portional to  the  pressure. 
This  phenomenon  is  more 
marked  the  denser  the  gas 
which  is  ionized.  The 

curves  shown  in  Fig.  57,  which  are  due  to  Rutherford,  illustrate 
very  clearly  this  general  phenomenon  in  the  case  of  three  dif 
ferent  gases.     The  dotted  line  shows  where  the  curves  would 
run  if  direct  proportionality  were  shown  over  the  whole  range 
of  pressure. 

This  effect  of  absorption  is  more  marked  the  greater  the 
distance  between  the  plates  C  and  D,  for  the  greater  the  dis- 
tance the  rays  have  to  travel  the  more  absorption  takes  place. 
If  the  plates  be  placed  only  5  or  6  mm.  apart  instead  of  4  cm. 
the  rays  are  absorbed  to  a  very  much  less  extent  and  the 
ionization  between  the  plates  is  much  more  nearly  uniform 
at  atmospheric  pressure  than  in  the  case  of  the  greater  distance. 
As  the  pressure  is  decreased  the  stage  at  which  the  ionization 
is  quite  uniform  is  reached  much  more  quickly  and  the  current 
is  much  more  nearly  proportional  throughout  the  range  of 
pressure. 

These  facts  just  described  may  be  very  easily  tested  experi- 
mentally by  means  of  the  apparatus  shown  in  Fig.  56  slightly 
modified.  Remove  the  lower  plate  CD  and  the  ebonite  rods 
and  insulate  the  plate  P.  The  latter  may  be  done  by  attaching 


170 


GENERAL   PROPERTIES   OF   RADIATIONS 


an  ebonite  block  to  the  under  side  of  P  into  which  the  rod  5 
fits.  Then  connect  P  to  the  battery  and  use  AB  and  P  as  the 
two  plates  between  which  the  ionization  occurs.  Place  these 
plates  about  4  cm.  apart  and,  using  a  thin  layer  of  uranium 
placed  on  the  plate  P,  measure  the  saturation  current  at  dif- 
ferent pressures  from  an  atmosphere  downwards.  Repeat  this 
when  the  plates  are  at  shorter  distances  apart.  Plot  the  curves 
on  the  same  scale,  showing  the  relation  between  current  and 
pressure  in  each  case  and  make  comparisons.  Repeat  this  for 
the  different  available  gases. 

If  the  /?  rays  are  used  as  the  ionizing  agent  the  ionization 
produced  will  be  much  more  nearly  proportional  to  the  pressure 
on  account  of  their  greater  penetrability. 

105.  Relation  Between  Current  and  Distance  Between  the 
Plates. — It  was  shown  (§56),  in  studying  Rontgen  rays,  that 
if  a  beam  of  Rontgen  rays  passed  between  two  parallel  plates 
so  as  to  produce  uniform  ionization  between  them  and  the 
saturation  current  measured  for  different  distances  apart  of 
the  plates  the  current  was  proportional  to  the  distance  between 
them.  The  same  is  true  for  the  a  rays  from  radio-active  bodies 
if  the  ionization  is  uniform.  The  absorption  of  the  latter  type 

of  rays,  however,  produces  in 
this  connection  a  similar  result 
to  that  described  in  the  last  para- 
graph. As  the  distance  between 
the  plates  increases  the  absorp- 
tion increases  and  consequently 
the  ionization  does  not  rise  as 
quickly  as  it  would  if  the  rays 
could  penetrate  the  whole  dis- 
tance without  suffering  any  ab- 
sorption. If  there  were  no  ab- 
sorption the  ionization  would 
increase  in  proportion  to  the 

distance  between  the  plates.  Since  the  absorption  is  greater 
at  higher  pressures  this  deviation  from  direct  proportionality 
is  more  marked  at  the  higher  pressures  than  at  the  lower  ones. 


a^ 


FIG.  58. 


RELATION  OF  CURRENT  TO  DISTANCE  BETWEEN   PLATES    171 

This  effect  is  well  illustrated  by  a  set  of  curves,  which  are 
due  to  Rutherford,  shown  in  Fig.  58.  At  the  lower  pressures 
the  current  is  much  more  nearly  proportional  to  the  distance 
than  at  the  higher  pressures.  These  effects  may  be  verified 
experimentally  by  use  of  the  same  apparatus  used  in  the 
experiments  of  the  last  paragraph.  Keeping  the  pressure 
fixed,  measure  the  current  for  different  distances  between  the 
plates  from  2  or  3  cm.  downwards  and  plot  a  curve  showing 
the  relation  between  current  and  distance  between  the  plates. 
Obtain  similar  curves  for  several  different  pressures.  Repeat 
these  experiments  also  in  different  gases  and  reducing  the 
observations  to  the  same  scale  compare  them. 

If  the  /?  rays  are  used  as  ionizers,  since  they  are  much  more 
penetrating  than  the  a  rays,  the  absorption  will  play  a  much 
less  important  part  and  the  current  due  to  the  ft  rays  will  be 
much  more  approximately  proportional  to  the  distance. 


CHAPTER  XL 

SOME  SPECIAL  PROPERTIES  AND  CONSTANTS 
OF   THE   RAYS. 

1 06.  Electric  Charge  Carried  by  (3  Rays. — The  magnetic  and 
electrostatic  deflection  of  both  the  a  and  (3  rays  show  that 
these  rays  must  consist  of  charged  particles  travelling  with 
high  velocity.  The  direction  of  deflection  shows  in  each  case 
the  sign  of  the  charge  carried.  If  these  rays  then  carry  a 
charge,  and  they  be  allowed  to  fall  upon  an  insulated  metal 
plate  the  plate  ought  to  receive  a  charge.  A  practical  difficulty 
immediately  arises  in  trying  to  demonstrate  this  experimen- 
tally, for  the  gas  surrounding  the  plate  becomes  ionized  by  the 
rays  and  conducts  the  charge  away  from  the  plate  as  fast  as 
it  is  received  and  therefore  no  resultant  charge  remains  to  be 
observed.  This  difficulty  may  be  overcome  in  the  case  of  the 
£  rays  by  a  special  method  used  by  M.  and  Mme.  Curie. 

In  this  experiment  a  heavy  metal  plate  AB  (Fig.  59)  was 
connected  to  an  electrometer  by  a  wire  C.  This  plate  and  wire 


FIG.  59. 

were  completely  surrounded  by  an  insulating  substance,  either 
ebonite  or  paraffin.  This  prevented  the  air  from  coming  in 
contact  with  the  plate  and  discharging  it.  This  insulation  was 
surrounded  by  a  metal  covering  connected  to  earth.  On  the 
lower  side  the  insulation  and  metal  covering  were  very  thin 
to  allow  the  rays  to  pass  through  without  much  absorption. 
The  metal  was  aluminium  foil  .01  mm.  thick  and  the  ebonite 

172 


was  .3  mm.  thick.  The  radio-active  material  R  was  contained 
in  a  hollow  cut  in  a  heavy  lead  block.  The  (3  rays  were  suffi- 
ciently penetrating  to  pass  through  the  covering  and,  falling 
upon  the  plate  AB,  gave  up  their  charge  to  it.  The  elec- 
trometer indicated  that  AB  received  a  negative  charge.  This 
charge  was  small,  but  could  be  measured  by  a  sensitive  elec- 
trometer. The  charge  must  have  been  directly  communicated 
to  the  plate  by  the  rays,  for  no  ionized  gas  could  come  in 
contact  with  the  plate.  From  other  considerations  Rutherford 
has  made  a  determination  of  the  number  of  p  particles  emitted 
per  second  by  one  gram  of  radium  bromide  and  has  found  it 
to  be  4  X  io10. 

107.  "  Radium  Clock." — A  very  simple  and  ingenious 
method  of  demonstrating  experimentally  that  the  /2  rays  carry 
a  negative  charge  has  been  devised  by  Strutt,  which,  on  account 
of  its  periodic  and  automatic  motions,  is  often  called  the 
"  radium  clock."  Since  the  /?  rays  carry  a  negative  charge, 
they  ought  to  leave  the  radium  or  the  substance  from  which 
they  come  positively  charged.  When  the  radium  is  exposed 
to  the  air  this  fact  cannot  be  observed, 
for  the  ionized  air  discharges  the  radium 
as  soon  as  it  is  charged,  and  besides  that 
the  positively  charged  a  particles  are 
also  emitted  simultaneously  with  the  J3 
particles,  and  they  would  leave  the  ra- 
dium negatively  charged  and  thus  tend 
to  counterbalance  the  effect  of  the  (3  rays. 
In  Strutt's  apparatus,  which  is  shown  in 
Fig.  60,  these  experimental  difficulties 
are  overcome.  A  sealed  tube  T  contains 
some  radium  which  is  in  metallic  contact 
with  two  gold  leaves  L  and  L.  Q  is  an 
insulating  quartz  rod.  The  tube  T  is 

made  thick  enough  to  completely  absorb  all  the  a  rays  but 
allows  the  (3  rays  to  pass  through,  and  therefore  the  a  rays 
with  their  positive  charge  do  not  escape  from  the  system.  This 
whole  system  is  enclosed  in  a  large  glass  tube  coated  on  the 


174  SPECIAL    PROPERTIES   OF   THE  RAYS 

inside  with  tinfoil  connected  to  earth  to  prevent  the  charging 
up  of  the  case.  The  air  is  pumped  out  as  completely  as  pos- 
sible to  prevent  conduction  through  the  gas,  so  that  any  charge 
acquired  by  the  central  system  may  not  be  lost  by  conduction. 
As  the  /?  rays  carrying  the  negative  charge  leave  the  tube  the 
insulated  system  connected  with  the  radium  becomes  positively 
charged  and  the  gold  leaves  diverge.  Two  metal  plates,  P  and 
P,  connected  to  earth,  are  placed  so  that  when  the  leaves  reach 
a  given  distance  apart  they  touch  the  plates  and  lose  their 
charge  and  collapse,  but  they  immediately  begin  to  charge  up 
again.  This  arrangement  will  work  automatically  at  a  prac- 
tically constant  rate,  depending  upon  the  activity  of  the  radium, 
for  a  number  of  years.  From  other  evidence  which  will  be 
deduced  later  we  have  reason  to  believe  that  this  rate  would 
change  after  a  very  large  number  of  years,  due  to  the  diminu- 
tion in  the  number  of  /?  particles  emitted. 

1 08.  Electric  Charge  Carried  by  a  Rays. — The  method  of 
§  106  cannot  be  satisfactorily  applied  to  the  a  rays  on  account 
of  their  very  weak  penetrating  power.  Any  covering"  which 
could  satisfactorily  be  used  as  a  shield  would  absorb  the  a  rays 
before  they  reached  the  plate  AB. 

Recently  Rutherford  and  Geiger  have  devised  a  very  novel 
and  ingenious  method  for  not  only  measuring  the  charge  car- 
ried by  the  a  particle,  but  actually  counting  the  number  of  a 
particles  emitted  by  radio-active  bodies.  The  method  used  by 
them  requires  very  careful  manipulation  and  is  attended  by 
some  experimental  difficulties  which  have  to  be  overcome  but 
cannot  be  given  in  detail  here.  The  following  is  the  general 
principle  of  their  method. 

The  amount  of  ionization  produced  by  a  single  a  particle  is 
very  small  and  would  be  very  difficult  to  measure  except  by  an 
exceedingly  sensitive  apparatus.  The  ionization  current  pro- 
duced by  a  single  a  particle  was  magnified  by  a  special  method 
depending  on  the  principle  of  the  production  of  ions  by  colli- 
sion (§67).  The  a  particles  were  allowed  to  pass  through  a 
very  small  opening  into  a  detecting  vessel  containing  gas  at.  a 
low  pressure  uj  which  an  electric  field  was  established  very 


CHARGE    CARRIED    BY   a    RAYS  1 75 

nearly  equal  to  the  value  required  to  produce  a  spark.  When 
an  a  particle  enters  this  strong  field  the  velocity  of  any  ions 
produced  is  so  increased  by  the  powerful  field  that  more  ions 
are  produced  by  collision  and  therefore  the  ionizing  effect 
of  the  a  particle  is  greatly  magnified.  The  entrance  of  an 
a  particle  is  therefore  marked  by  a  sudden  throw  of  the 
electrometer  needle  due  to  this  sudden  production  of  ions.  By 
careful  adjustment  of  the  electric  field,  etc.,  they  were  able  to 
detect  the  entrance  of  a  single  particle  and  therefore  to  actually 
count  the  number  entering  in  a  given  time  by  noting  the  num- 
ber of  throws  of  the  needle.  By  this  method  they  determined 
that  the  total  number  of  a  particles  expelled  per  second  from 
one  gram  of  radium  in  equilibrium  is  1.36  X  lo11. 

In  this  connection  they  also  observed  by  the  eye  the  number 
of  scintillations  produced  when  the  a  particles  fell  upon  a 
phosphorescent  screen  of  zinc  sulphide.  The  number  of  scin- 
tillations was  found  to  be  the  same  as  the  number  of  imping- 
ing a  particles  counted  by  the  electrical  method.  So  the  a  par- 
ticles may  be  counted  by  either  the  electrical  or  scintillation 
method. 

These  experiments  on  the  a  particles  mark  a  wonderful 
advance  in  modern  experimental  methods,  and  are  especially 
noteworthy  as  this  is  the  first  instance  in  which  a  single  iso- 
lated atom  of  matter  has  been  independently  detected  and 
measured  in  any  way.  This  is  possible,  of  course,  simply  on 
account  of  the  great  energy  possessed  by  the  a  particle.  We 
shall  see  later  that  the  a  particle  is  really  an  atom  of  helium 
carrying  a  charge,  and  therefore  a  single  atom  may  be  isolated 
by  this  method. 

During  this  series  of  experiments  on  the  a  particles  the 
charge  carried  by  each  particle  was  experimentally  determined. 
Since  the  number  of  a  particles  emitted  per  second  by  a  known 
quantity  of  radium  is  known,  the  charge  carried  by  each  one 
may  easily  be  found  if  the  total  charge  carried  by  the  known 
number  of  particles  be  measured.  This  quantity  was  measured 
and  it  was  found  that  each  a  particle  carried  a  positive  charge 
of  9.3  X  icr10  electrostatic  units. 


176  SPECIAL    PROPERTIES    OF   THE  RAYS 

109.  Velocity  and  Value  of  e/m  for  ft  Rays. — The  devia- 
bility  of  the  ft  rays  by  a  magnetic  and  an  electrostatic  field 
makes  it  possible  to  experimentally  determine  the  velocity  of 
these  particles  and  the  ratio  of  the  charge  to  the  mass  by  a 
method  similar  in  principle  to  that  used  for  the  same  purpose 
for  cathode  rays  described  in  §  36.  Becquerel  used  this  prin- 
ciple and  allowed  a  narrow  beam  of  rays  to  fall  upon  a  photo-- 
graphic plate  and  observed  the  deviation  produced  by  known 
magnetic  and  electrostatic  fields.  He  found  the  average  veloc- 
ity to  be  about  1.6  X  io10  cm.  per  second.  The  velocity  of 
cathode  rays  we  have  seen  (§36)  is  2.8  X  io9  cm.  per  second 
so  the  velocity  of  the  ft  rays  is  considerably  greater  than  that 
of  the  cathode  rays. 

As  we  have  previously  noted  the  ft  rays  from  radium  are 
complex.  This  was  shown  in  Becquerel's  experiments  by  the 
fact  that  some  of  the  rays  are  bent  more  than  others  by  the 
same  field.  He  found  that  the  velocities  varied  from  about 
6  X  io9  to  2.8  X  io10  cm.  per  second.  The  latter  velocity 
approaches  very  nearly  the  velocity  of  light,  which  is  3  X  i°10 
cm.  per  second. 

Using  the  same  rays,  which  had  a  velocity  of  i.6Xio10 
cm.  per  second,  Becquerel  determined  the  value  of  e/m  and 
found  it  to  be  io7.  This  does  not  differ  much  from  the  value 
found  by  J.  J.  Thomson  (§36)  for  the  cathode  ray  particle 
which  indicates  that  the  ft  particle  is  similar  to  the  cathode 
ray  particle  carrying  the  same  charge  and  of  about  the 
same  mass. 

This  complexity  of  the  ft  rays  with  regard  to  velocity  led 
Kaufmann  to  examine  whether  the  value  of  e/m  for  these 
rays  varied  with  the  speed.  He  showed  experimentally  that 
the  value  of  e/m  decreased  when  the  speed  increased.  If  we 
make  a  most  probable  assumption  that  the  charge  on  the  ft 
particle  is  constant  the  mass  then  appears  to  increase  with  the 
increase  of  velocity.  It  has  been  demonstrated  from  purely 
theoretical  considerations  by  several  mathematical  physicists 
that  the  apparent  mass  of  a  moving  electron  is  due,  either 
wholly  or  in  part,  to  the  electric  charge  in  motion,  that  is, 


VELOCITY   OF   THE   RAYS  1 77 

when  an  electric  charge  is  in  motion  it  appears  to  possess  what 
corresponds  to  inertia  due  to  the  fact  of  its  being  in  motion. 
This  apparent  inertia,  according  to  this  view,  is  not  due  to 
material  mass  as  we  are  accustomed  to  conceive  of  it  but  is  a 
direct  result  of  the  motion  of  the  electric  charge.  These  theo- 
retical considerations  further  show  that  this  apparent  mass, 
which  appears  to  be  electrical  in  origin,  increases  with  the 
speed  of  the  moving  charge.  The  experimental  results  of 
Kaufmann  appear  to  confirm  the  theoretical  view  that  the 
mass  of  the  electron  is  due,  wholly  or  in  part,  to  the  fact  that 
the  electric  charge  is  in  motion. 

no.  Velocity  and  Value  of  e/m  for  a  Rays. — The  deter- 
mination of  the  velocity  of  projection  of  the  a  particles  and 
the  ratio  of  their  charge  to  their  mass  is  much  more  difficult 
experimentally  than  the  corresponding  determination  for  the 
/?  particles  on  account  of  the  comparatively  small  deflection 
produced  by  even  quite  powerful  electrostatic  and  magnetic 
fields  and  very  special  methods  have  to  be  used.  Several  deter- 
minations of  these  values  have  been  made,  but  the  most  recent 
and  accurate  determinations  have  been  made  by  Rutherford 
who  used  the  following  method. 

It  is  important  that  as  homogeneous  and  as  active  a  source  of 
rays  as  possible  be  used  for  this  purpose.  Radium,  for  reasons 
which  will  be  explained  later,  although  strongly  active,  gives 
out  a  complex  set  of  a  rays.  Rutherford  therefore  used  as 
his  source  of  rays  a  very  active  deposit  of  radium  C  on  a  thin 
wire  about  0.5  mm.  in  diameter.  (The  explanation  of  and 
method  of  depositing  radium  C  will  be  given  in  Chapter  XIV.) 
This  source  of  rays  possesses  several  advantages.  The  rays 
are  homogeneous  in  character ;  they  suffer  no  absorption  by 
the  active  material,  as  tfie  layer  of  active  material  on  the  wire 
is  so  extremely  thin ;  also  the  source  is  very  small  and  sharply 
defined  as  the  deposit  can  be  made  on  a  very  thin  wire. 

The  apparatus  used  in  the  case  of  the  magnetic  deflection 
is  shown  in  Fig.  61.  The  wire  with  the  active  material  on  it 
was  placed  in  a  groove  V  at  a  distance  of  2  cm.  below  a  narrow 
slit  5\  The  rays  passed  through  this  slit  and  then  fell  upon 


78 


SPECIAL    PROPERTIES    OF   THE   RAYS 


FIG.  61. 


a  small  photographic  plate  P  which  was  supported  at  a  fixed 
distance  of  some  4  or  5  cm.  above  the  -slit.  Over  the  whole 
system  a  brass  tube  T  was  placed  from  which  the  air  could 
be  rapidly  exhausted.  The  removal  of  the  air  is  important  as  it 
lessened  the  absorption  of  the  rays  which  are  so  easily  absorbed, 
and  therefore  greatly  increased  the  intensity 
at  the  photographic  plate.  Besides  this  the 
diminishing  of  the  absorption  allows  the 
plate  to  be  placed  farther  from  the  source 
so  that  a  larger  movement  of  the  photo- 
graphic impression  is  produced.  This  tube 
and  enclosed  system  was  placed  between  the 
rectangular  poles  of  a  very  powerful 
magnet  so  that  the  uniform  magnetic  field 
extended  from  a  distance  of  i  cm.  below  the 
slit  to  the  top  of  the  tube  T.  The  magnetic 
field  was  applied  for  a  given  time  in  one  direction  and  then  re- 
versed so  as  to  deflect  the  rays  in  the  opposite  direction.  The 
field  was  thus  reversed  every  ten  minutes  for  a  period  of  about 
one  hour.  This  was  done  so  that  the  distance  between  the 
photographic  impressions  on  the  plate  would  be  double  of  what 
the  distance  would  be  if  the  field  were  applied  in  only  one  direc- 
tion. The  strength  of  magnetic  field  used  was  about  9470 
C.G.S.  units,  and  this  would  produce  a  separation  between  the 
bands  of  4.7  mm.  when  the  photographic  plate  was  4  cm. 
from  the  slit. 

The  path  of  the  deflected  beam  of  rays  constitutes  a  curve 
whose  radius  of  curvature  may  be  easily  calculated  from  the 
known  dimensions. 

Let  2r  —  the  distance  between  the  centres  of  the  bands  on 

the  photographic  plate, 

PI  =  the  distance  between  the  plate  P  and  the  slit  S, 
r2  =  the  distance  of  the  slit  S1  above  the  beginning  of 

the  magnetic  field. 

Then  since  the  curvature  is  small  the  radius  of  curvature  p  is 
given  by  the  equation 


2p  x  r  = 


VELOCITY   OF   THE   3   RAYS 


179 


therefore 


M 


It  has  been  previously  shown  (§36)  that  when  a  charged 
body  of  mass  m  and  carrying  a  charge  e  is  in  motion  with  a 
velocity  v  in  a  magnetic  field  of  strength  H  the  radius  of 
curvature  of  the  path  is  given  by  the  equation  Hp  =  mv/e. 
The  values  of  H  and  p  are  determined  and  therefore  nw/e 
is  known. 

The  apparatus  used  for  the  electrostatic  deflection  of  the 
rays  is  shown  in  Fig.  62.  A 
similar  source  of  radiation 
was  placed  in  a  groove  at  W . 
Two  insulated  parallel  plates 
A  and  B  about  4  cm.  high  and 
0.21  mm.  apart  formed  a  very 
narrow  slit  for  the  rays  to 
pass  through.  After  passing 
through  this  narrow  slit  the 
rays  fell  upon  a  photographic 
plate  P.  The  whole  was  en- 
closed by  the  brass  tube  M 
which  could  be  quickly  ex- 
hausted. The  deflecting  potential  was  applied  between  the 
plates  A  and  B,  the  former  being  in  metallic  contact  with 
the  enclosing  vessel  while  the  latter  was  insulated  from  it. 
This  deflecting  potential  was  applied  first  in  one  direction 
and  then  in  the  other  at  intervals  so  that  a  double  deflection 
of  the  rays  was  the  result. 

The  mathematical  demonstration  of  the  theory  of  this  experi- 
ment is  somewhat  lengthy  and  more  complicated  that  in  the 
case  of  the  magnetic  deflection,  and  consequently  will  not  be 
given  in  detail  here.  If  desired  it  may  be  found  in  the  original 
paper*  by  Rutherford.  If  the  height  of  AB  is  h^  the  distance 
from  the  top  of  B  to  the  plate  P  is  h2,  the  distance  between  the 
plates  is  d,  the  distance  between  the  extreme  edges  of  the 

*Phil.  Mag.,  Oct.,  1906, 


l8o  .  SPECIAL    PROPERTIES   OF   THE  RAYS 

photographic  bands  for  a  reversal  of  the  electric  field  is  d: 
and  the  deflecting  potential  F'then,  from  the  theory  under 
proper  conditions,  the  following  equation  is  true,  namely, 


mv2          8  F//22 


\2' 


which  gives  the  value  of  mv^/e.  Combining  this  value  with 
the  value  of  nw/e  obtained  from  the  magnetic  deflection  the 
value  of  v  and  of  e/m  are  determined.  The  latest  results  ob- 
tained by  Rutherford  and  other  experimenters  show  that  the 
value  of  e/m  is  the  same  for  the  a  rays  emitted  by  the  various 
radio-active  substances  and  is  equal  to  5  X  io3  in  electro- 
magnetic units. 

Although  this  quantity  is  constant  the  velocity  of  expulsion 
of  the  a  particles  is  not  the  same  for  all  substances.  It  is 
found  to  vary  from  about  1.56  X  io9  to  2.25  x  io9  cm.  per 
second  under  different  circumstances. 

in.  Mass  and  Nature  of  the  a  Particle. — These  results 
along  with  others  enable  us  to  obtain  a  more  definite  idea  of 
the  mass  of  the  a  particle  and  consequently  of  its  true  nature. 
The  value  of  E/M  for  the  atom  of  hydrogen  liberated  in  the 
electrolysis  of  water  is  in  round  numbers  io4  electromagnetic 
units.  The  charge  E  carried  by  the  hydrogen  atom  is  believed 
to  be  the  smallest  fundamental  charge  carried  by  any  existing 
particle  of  matter  so  that  the  charge  carried  by  any  body  must 
be  an  integral  multiple  of  E.  The  charge  carried  by  a  hydro- 
gen ion  is  equal  to  the  charge  carried  by  a  gaseous  ion, 
which  was  found  (§81)  to  be  3.4  X  io~10  electrostatic  units. 
Rutherford  has  recently  shown  however  that  certain  experi- 
mental errors  in  the  determination  of  this  charge  carried  by 
the  gaseous  ion  tend  to  make  this  value  too  small  and  it  is 
most  probably  in  the  neighborhood  of  about  4.6  X  io~10.  We 
have  also  seen  (§  108)  that  the  charge  carried  by  the  a  particle 
is  equal  to  9.3  X  icf 10  electrostatic  units.  It  follows  then  from 
this  that  the  a  particle  carries  twice  the  charge  carried  by  the 
hydrogen  atom.  This  being  so  it  follows,  since  E/M=iQ* 


ENERGY   OF   THE   a    PARTICLE  iSl 

and  e/m  for  the  a  particle  is  equal  to  5  x  io3  and  e  =  2E, 
that  the  mass  of  the  a  particle  must  be  four  times  the  mass  of 
the  hydrogen  atom  and  therefore  must  be  atomic  in  size.  Now 
the  atomic  mass  of  helium  is  3.96  in  terms  of  hydrogen.  Thus 
we  see  that  the  a  particle  is  atomic  in  size  and  of  the  order 
of  the  helium  atom,  and  therefore  must  take  its  place  among  the 
elements.  But  since  there  does  not  seem  to  be  any  place  ac- 
cording to  the  periodic  law  among  the  elements  for  a  new  one 
in  that  part  of  the  series  the  most  probable  hypothesis  is  that 
the  a  particle  is  an  atom  of  helium  carrying  twice  the  charge 
of  a  hydrogen  atom.  The  a  particle  when  its  charge  is  neu- 
tralized becomes  an  ordinary  uncharged  helium  atom.  This 
theory  is  further  supported  by  the  fact  that  helium  is  very 
commonly  found  along  with  old  radio-active  minerals. 

112.  Energy  of  the  a  Particle. — Since   the  a   particle  is 
atomic  in  size  and  is  travelling  with  high  speed  it  must  possess 
considerable  kinetic  energy.     It  will  be  of  interest  to  calculate 
this  energy,  which  may  be  easily  done  by  means  of  the  results 
just  obtained.     The  kinetic  energy  of  a  mass  m  moving  with 
a  velocity  v  is  equal  to  \nvvi  =  \m/e  -  v*  -  e.      The  value  of 
m/e  is  i/ (5  X  io3)  electromagnetic  units.    The  average  value 
of   v    is    practically    2  X  io9    cm.    per  sec.     The  charge  e  is 

Q    O        )(        IO~^ 

o.-?  x  io~10  electrostatic  units  which  equals  —  —  —  electro- 

3  x  io10 

magnetic  units,  which  equals  3.1  X  io"20.  Supplying  these 
values  in  the  above  expression  we  have  the  average  kinetic 
energy  of  each  moving  a  particle  equal  to 

2*5  XIio3X(2X  IC)9)2x(3-i  x  io-20)=  12-4  x  io-6ergs. 

113.  Nature  of  the  y  Rays. — The  y  rays  differ  very  essen- 
tially from  the  a  and  /?  rays.     They  are,  as  we  have  seen, 
extremely   penetrating   compared   with   the   other  two   types. 
Very  active  radium  bromide  emits  y  rays  which  can  be  detected 
after  passing  through  as  much  as  30  cm.  of  iron.     There  is  a 
still  greater  essential  difference  in  the  fact  that  no  one  has  as 
yet  succeeded  in  deviating  the  y  rays  by  either  a  magnetic  or 


1 82  SPECIAL    PROPERTIES    OF   THE   RAYS 

electric  field.  They  do  not  appear  to  carry  any  electric  charge 
at  all.  It  has  consequently  been  very  difficult  to  determine 
the  real  nature  of  the  y  rays  by  direct  experiment.  Their  great 
penetrating  power  and  non-deviability  show  a  strong  resem- 
blance to  very  hard  Rontgen  rays.  Two  rival  theories  as  to 
the  nature  of  y  rays  have  been  put  forward,  each  of  which 
has  a  considerable  amount  of  experimental  evidence  in  its 
favor,  but  up  to  the  present  neither  has  been  thoroughly 
established. 

We  know  that  Rontgen  rays  are  produced  by  the  sudden 
stopping  of  a  moving  electron,  and  it  is  reasonable  to  suppose 
that  they  would  be  produced  by  the  sudden  starting  of  an 
electron  into  rapid  motion.  Now  experiment  has  shown  that 
y  rays  always  occur  in  conjunction  with  rapidly  moving  (3 
particles  which  we  know  are  electrons.  It  is  therefore  reason- 
able to  suppose  that  the  y  rays  are  electromagnetic  pulses  simi- 
lar to  Rontgen  rays  produced  by  the  sudden  expulsion  of  the 
y8  particles,  or  electrons,  from  the  radio-active  substance.  This 
theory  has  a  large  amount  of  evidence  both  of  a  theoretical 
and  experimental  nature  to  support  it. 

Another  theory  has  recently  been  advanced  by  Bragg  to  the 
effect  that  these  rays,  instead  of  being  of  the  nature  of  a 
vibration,  are  of  a  material  nature.  He  suggests  that  they 
consist  of  neutral  pairs  of  positively  and  negatively  charged 
particles.  Their  neutral  nature  would  account  for  the  non- 
deviability  by  a  magnetic  or  electric  field.  He  has  also  de- 
duced considerable  experimental  proof  in  favor  of  this  theory. 
Although  the  balance  of  proof  at  present  seems  to  be  in  favor 
of  the  electromagnetic  pulse  theory,  yet  neither  theory  has  been 
satisfactorily  proved  or  disproved  and  further  experimental 
data  are  required  on  this  subject. 


CHAPTER  XII. 
URANIUM   X,   THORIUM   X,  ACTINIUM   X. 

114.  Discovery  of  Uranium  X  and  Thorium  X. — In  the  year 
1900  Sir  William  Crookes  showed  that  by  a  simple  chemical 
process  he  could  separate  from  uranium  a  constituent  which 
was  many  times  more  active  photographically  than  the  ura- 
nium from  which  it  was  separated.     In  addition,  the  separa- 
tion of  this  constituent  left  the  uranium  photographically  in- 
active.    This  new  and  unknown  substance  he  called  uranium 
X,   or   Ur.   X.      Becquerel   obtained   similar   results,   using  a 
slightly  different  chemical  process,  and  in  addition  he  discov- 
ered the  curious  fact  on  testing  some  months  later  the  uranium 
X  and  the  uranium  from  which  it  had  been  separated  that  the 
uranium  had  completely  recovered  its  usual  amount  of  activity 
while  the  Ur.  X  had  entirely  lost  its  activity.     Rutherford  and 
Soddy  later  succeeded  in  performing  a  similar  chemical  opera- 
tion on  thorium,  separating  a  very  active  constituent  which 
they  called  thorium  X,  or  Th.  X,  and  which  acted  in  a  manner 
very  similar  to  Ur.  X.     We  will  now  study  these  actions  a  little 
more  in  detail. 

115.  Chemical  Separation  of  Ur.  X. — Dissolve  a  few  grams 
of  uranium  nitrate  in  water  and  then  add  just  sufficient  am- 
monium carbonate  to  precipitate  the  uranium  from  the  solution. 
This  precipitate  contains  the  uranium  X  as  well  as  the  uranium. 
If  an  excess  of  the  ammonium  carbonate  be  added  the  uranium 
will  be  redissolved,  but  the  uranium  X  will  not  be  dissolved  and 
will  be  left  behind  as  an  insoluble  precipitate.     Separate  this 
insoluble  precipitate  from  the  solution  and  dry  it.     Also  evapo- 
rate the  dissolved  uranium  solution  to  dryness.     The  Ur.  X 
and  uranium  residue  are  now  entirely  separated. 

116.  Activity  of  Uranium  and  Ur.  X. — Using  a  small  testing 
vessel  of  the  form  shown  in  Fig.  50  test  the  uranium  residue 
in  the  usual  manner  for  a  ray  and  ft  ray  activity  separately. 

183 


184  URANIUM    X,   THORIUM    X,   ACTINIUM    X 

It  will  be  found  that  the  uranium  residue  has  entirely  lost  its 
(3  ray  activity  but  that  its  a  ray  activity  is  undiminished.  Test 
in  the  same  way  the  uranium  X  precipitate  and  observe  that  it 
emits  no  a  rays  at  all  but  gives  out  a  strong  ft  radiation.  If 
these  two  precipitates  be  tested  photographically  it  will  be 
found  that  the  radiations  from  Ur.  X  will  produce  a  strong 
impression  on  a  photographic  plate,  but  that  the  uranium  resi- 
due is  practically  inactive  photographically. 

These  results  furnish  an  excellent  illustration  of  the  differ- 
ence between  the  electrical  and  photographic  methods  of  test- 
ing radio-active  bodies.  Tested  electrically  the  uranium  res- 
idue is  quite  active  because  it  gives  out  only  a  rays  which  are 
strong  ionizers,  while  the  Ur.  X  is  practically  inactive,  as  it 
gives  off  only  ft  rays,  which  are  very  weak  ionizers.  Tested 
photographically  the  opposite  result  is  obtained,  namely,  that 
the  uranium  residue  is  practically  inactive,  because  the  a  rays 
which  it  emits  produce  very  little  photographic  effect,  while  the 
Ur.  X  is  very  active  because  the  ft  rays  are  strongly  active 
photographically.  Great  care  must  therefore  be  observed  in 
comparing  measurements  made  by  the  two  different  methods, 
to  avoid  confusion,  for  the  two  methods,  as  we  see,  give  in 
some  cases  entirely  opposite  results. 

117.  Change  in  Activity  of  Uranium  and  Ur.  X. — Starting 
immediately  after  the  chemical  separation  of  uranium  and  Ur. 
X  test,  by  the  electrical  method,  both  substances  for  both 
a  and  ft  ray  activity  separately  and  repeat  these  tests  at  inter- 
vals of  about  once  a  day  for  a  period  of  from  75  to  100  days, 
or  even  longer  if  time  will  permit.  The  curious  fact  will  be 
observed  that  the  a  ray  activity  of  the  uranium  residue  will 
remain  constant,  but  the  ft  ray  activity  gradually  increases  with 
the  time  and  finally  reaches  a  maximum  and  then  remains  con- 
stant. The  ft  ray  activity  may,  of  course,  be  measured  sepa- 
rately by  cutting  off  the  a  rays  in  the  usual  manner.  It  takes 
it  a  little  over  five  months  to  reach  this  maximum.  The  ft  ray 
activity  of  the  Ur.  X,  on  the  other  hand,  gradually  decreases 
with  the  time,  and  not  only  so  but  decreases  at  exactly  the  same 
rate  that  the  ft  ray  activity  of  the  uranium  residue  increases. 


THORIUM    AND   THORIUM    X  185 

Plot  a  curve  in  each  case  showing  the  relation  between  the  time 
and  the  ft  ray  activity  as  measured  by  the  ionization  current. 
Two  such  curves,  due  to  Rutherford  and  Soddy,  representing 
this  decay  and  recovery  of  activity  are  shown  in  Fig.  63,  in 
which  the  ordinates  represent  the  activity  as  measured  by  the 
ionization  current,  while  the  abscissae  represent  the  time  in 


FIG.  63. 

days  after  chemical  separation.  These  curves  represent  in  both 
cases  the  activity  as  measured  by  the  (3  rays.  It  will  be  ob- 
served that  the  time  required  for  the  activity  in  the  one  case 
to  rise  to  half  its  maximum  value  is  equal  to  the  time  required 
in  the  other  to  fall  to  half  its  maximum  value  and  that  this 
period  is  equal  to  22  days. 

118.  Thorium  and  Th.  X. — Make  a  dilute  solution  of  thor- 
ium nitrate  in  water.  Then  add  sufficient  ammonia  to  precipi- 
tate the  thorium  as  thorium  hydroxide.  Separate  the  precipi- 
tate and  carefully  dry  it.  Also  evaporate  the  filtrate  to  dryness 
and  remove  the  ammonium  salts  from  the  residue  by  ignition. 
The  remainder  will  consist  of  a  very  active  substance  many 
times  more  active  weight  for  weight  than  the  thorium  salt 
which  was  first  dissolved.  This  active  constituent  has  been 
named  thorium  X,  or  Th.  X,  from  analogy  with  uranium  X. 
Test  in  the  usual  way,  as  soon  after  preparation  as  possible, 
the  activity  of  the  precipitated  thorium  hydroxide  and  also  of 
the  Th.  X,  and  repeat  the  tests  at  intervals  of  about  three 


1 86  URANIUM    X,   THORIUM    X,    ACTINIUM    X 

times  a  day  for  a  period  of  about  twelve  or  fourteen  days. 
Observe  that  the  precipitated  thorium  has  lost  a  large  propor- 
tion of  its  original  activity  which  it  had  before  solution, 
although  it  has  not  lost  all  its  activity.  On  being  tested  at 
intervals,  this  activity  at  first  decreases  for  a  short  time  and 
then  begins  to  rise  again  and  continues  to  rise  until  it  reaches 
a  maximum,  when  it  remains  constant.  Test  also  simulta- 
neously the  activity  of  the  thorium  X  at  intervals  and  observe 
that  at  first  it  increases  for  a  short  time  and  then  begins  to 
decrease  and  continues  to  decrease  until  it  finally  disappears. 
Plot  a  curve  in  each  case,  showing  the  relation  between  activity 
and  time  after  chemical  separation.  With  the  exception  of  the 
initial  irregularity,  which  will  be  accounted  for  in  a  later  chap- 
ter, the  curves  obtained  should  be  very  similar  to  those  obtained 
in  the  case  of  uranium  and  Ur.  X.  Note  also  that  the  thorium 
recovers  its  activity  at  practically  the  same  rate  as  the  activity 
of  the  Th.  X  decays.  The  time  required  for  the  activity  of  the 
thorium  to  reach  half  its  maximum  will  be  seen  to  be  only 
about  four  days,  while  the  same  time  is  required  for  the  activity 
of  the  Th.  X  to  decay  to  half  its  maximum  value. 

119.  Actinium  and  Act.  X. — If  a  solution  of  an  actinium  salt 
be  made  and  treated  with  ammonia  in  just  the  same  manner  as 
the  thorium,  a  very  active  constituent  will  be  obtained  showing 
properties  very  similar  to  those  of  Th.  X.  This  substance  has 
been  named  actinium  X,  or  Act.  X,  from  analogy.  If  a  quantity 
of  the  so-called  emanium,  which  is  the  same  as  actinium,  is 
available,  make  a  solution  of  it  and  treat  it  in  the  same  way 
as  the  thorium  was  treated.  Test  the  activity  of  the  precipitate 
and  the  evaporated  residue  or  Act.  X  at  intervals  of  once  or 
twice  a  day  for  a  period  of  forty  or  fifty  days  and  observe  the 
recovery  of  activity  of  the  precipitate  and  the  decay  of  the 
activity  of  the  Act.  X  and  plot  curves  for  them  as  before. 
Curves  of  a  shape  almost  exactly  similar  to  those  obtained  for 
thorium  should  result  from  the  tests.  The  time  required  for 
the  actinium  to  regain  half  its  maximum  activity  and  also  for 
the  activity  of  Act.  X  to  decay  to  half  of  its  maximum  value 
will  be  found  to  be  about  ten  days. 


THEORY   OF   SUCCESSIVE   CHANGES  187 

1 20.  Theory  of  Successive  Changes. — These  results  indicate 
that  some  process  must  be  continually  going  on  in  these  sub- 
stances unaided  by  any  outside  agencies.  Since  the  Ur.  X, 
for  instance,  which  gives  out  ft  rays  can  be  separated  from  the 
normal  uranium,  leaving  it  devoid  of  ft  rays,  therefore  the 
ft  rays  must  arise  from  the  Ur.  X,  and  since  the  uranium  regains 
the  ft  ray  activity  after  being  deprived  of  Ur.  X,  more  Ur.  X 
must  be  formed  in  the  uranium  compound  to  give  rise  to  the 
ft  rays.  This  can  easily  be  shown  to  be  true,  for  after  the 
uranium  residue  has  recovered  its  activity  Ur.  X  can  be  sepa- 
rated a  second  time  from  the  compound  and  the  action  will  be 
repeated.  This  separation  may  be  repeated  as  often  as  desired 
after  recovery,  showing  a  continuous  production.  In  addition 
the  activity  of  the  Ur.  X  is  not  permanent,  but  gradually  dies 
away.  Also  we  know  that  the  ft  ray  activity  of  normal  uran- 
ium, which  contains  Ur.  X  from  which  the  ft  rays  arise, 
does  not  change,  consequently  there  must  be  a  state  of  equi- 
librium in  normal  uranium  in  which  fresh  Ur.  X  is  being 
formed  at  the  same  rate  as  it  dies  away  in  order  that  the  total 
resultant  activity  may  remain  constant.  This  is  borne  out  by 
the  fact  that  when  the  Ur.  X  is  isolated  from  the  normal  com- 
pound the  rate  of  decay  of  the  separated  Ur.  X  is  equal  to  the 
rate  of  recovery  of  the  uranium  from  which  it  was  separated. 
In  order  to  maintain  this  equilibrium  state  these  processes 
must  therefore  be  going  on  continuously  at  a  constant  rate,  and 
since  outside  agencies  do  not  affect  these  processes  the  cause 
of  the  action  must  arise  within  the  substances  themselves. 

If  the  decay  curve  for  Ur.  X  be  studied  it  will  be  observed 
from  the  form  of  the  curve  that  the  activity  decays  according 
to  an  exponential  law.  If  70  represent  the  initial  activity 
immediately  after  separation  and  It  the  activity  after  a  time  t, 
then  it  will  be  found  that  /f  =  /oC-x<,  where  e  is  the  natural 
base  of  logarithms  and  A  a  constant  quantity.  Similarly  the 
curve  representing  the  recovery  of  activity  by  the  uranium 
from  which  the  Ur.  X  has  been  separated  can  be  represented 
by  the  equation  It  =  I0(i — e~Xf),  where  70  represents  the 
activity  when  the  maximum  is  reached  and  It  is  the  activity 


1 88  URANIUM    X,   THORIUM    X,   ACTINIUM    X 

recovered  at  any  time  t  and  A  the  same  constant.  The  total 
activity  then  at  any  time  is  a  constant. 

The  results  of  a  large  variety  of  experiments  have  shown 
that,  as  far  as  tests  have  been  made,  neither  the  rate  of  decay 
of  the  separated  product  nor  the  rate  of  recovery  of  activity 
of  the  substance  from  which  it  was  separated  is  affected  by 
the  existing  physical  or  chemical  conditions  such  as  tempera- 
ture, etc.  After  chemical  separation  has  taken  place  the  re- 
covery of  activity  in  one  case  or  the  decay  of  activity  in  the 
other  proceeds  quite  independently  of  any  physical  or  chemical 
influences,  to  which  the  substances  may  be  subjected.  This 
indicates  that  the  process  arises  from  some  cause  within  the 
radio-active  substance  itself. 

Similar  processes  must  of  course  be  going  on  in  the  thorium 
and  actinium  compounds.  As  will  be  seen  later  conditions  of 
a  similar  character  but  differing  in  some  particular  exist  in 
the  radium  compounds. 

In  the  succeeding  chapter  it  will  be  seen  that  thorium  X,  for 
instance,  is  not  lost  when  its  activity  completely  decays,  but 
it  disappears  as  thorium  Xj  and  in  so  doing  changes  into 
another  product  or  substance  which  in  its  turn  gives  out  radio- 
active radiations.  These  facts  along  with  a  great  deal  of  addi- 
tional evidence,  some  of  which  will  be  considered  later,  led  to 
the  theory  of  successive  changes  in  radio-active  substances 
which  was  formulated  by  Rutherford  and  Soddy  to  explain 
these  curious  phenomena.  According  to  this  theory  the  differ- 
ent radio-active  substances  are  gradually  undergoing  a  process 
of  transformation  by  which  they  are  gradually  changing  in 
regular  succession  from  one  product  to  another  without  the 
help  of  any  outside  agency.  The  actual  amount  of  material 
transformed  in  each  instance  is  extremely  small,  but  the  elec- 
trical manifestations  are  so  pronounced  that  they  may  be  de- 
tected in  very  minute  quantities.  Most  of  these  products  give 
out  radiations.  In  some  cases  only  a  rays  are  emitted  and  in 
others  only  ft  and  y  rays,  while  in  others  all  three  types  are 
produced.  The  rates  at  which  these  changes  take  place  vary 
very  greatly  for  the  different  products,  some  changes  requiring 


THEORY    OF    SUCCESSIVE    CHANGES  189 

only  a  few  seconds  to  complete  while  others  extend  over  a 
period  of  several  hundred  years!  The  time  taken  by  any  one 
of  these  changes  to  be  half  completed  is  designated  the  period 
of  that  transformation  or  change.  The  reason  for  calling  half 
the  time  of  a  complete  transformation  the  period  instead  of 
the  whole  time  is  that  it  is  usually  much  more  convenient  to 
determine  experimentally  when  the  change  is  half  completed 
than  when  it  is  fully  completed,  for  during  the  latter  part  of 
the  transformation  the  rate  is  usually  much  slower  than  in  the 
earlier  stages,  as  will  be  observed  from  the  curves  already 
studied.  As  this  rate  of  change  in  the  later  stages  is  often 
so  slow  it  is  sometimes  difficult  to  determine  the  exact  time 
when  it  is  just  completed,  but  this  difficulty  is  not  so  likely 
to  occur  in  determining  when  the  transformation  is  half  com- 
pleted. This  theory  is  called  the  theory  of  successive  changes, 
and  the  different  products  into  which  each  one  is  gradually 
transformed  are  called  transformation  products. 


CHAPTER  XIII. 
EMANATIONS. 

121.  Discovery  of  Thorium  Emanation. — The  early  experi- 
menters  on   the    radiations   emitted   by   thorium    compounds 
observed  a  marked  difference  between  thorium  and  uranium, 
namely,  that  while  the  radiations  from  uranium  were  very  con- 
stant  those   produced    from   thorium   compounds   were   very 
irregular,   and   anything   like   constant   results   could   not   be 
obtained    from   them.     This   want   of   constancy  was   finally 
traced  to  the  presence  of  air  currents.     If  the  thorium  com- 
pound was  placed  in  a  closed  vessel  free  from  air  currents 
the  ionization,  although  for  a  few  minutes  at  first  was  some- 
what irregular,  soon  became  steady  and  would  remain  so,  but 
if  a  current  of  air  were  passed  through  the  vessel  the  ioniza- 
tion would  be  greatly  reduced.     If  the  ionization  were  tested 
in  an  open  vessel  where  it  was  subject  to  air  currents  the 
ionization  would  be  quite  unsteady.    This  irregularity  was  very 
thoroughly  investigated  by  Rutherford  and  he  found  that  it  was 
due  to  the  continuous  emission  of  some  sort  of  radio-active  par- 
ticles from  the  thorium  compounds.     To  these  particles  the 
name  emanation  was  given.     This  emanation  is  not  like  the 

adiations  which  we  have  already  considered,  but  it  acts  in  all 
•espects  like  an  ordinary  gas,  which  itself  emits  rays  of  the 
same  type  as  we  have  been  studying.  It  does  not  itself  con- 
sist of  ions  nor  charged  particles  of  any  sort,  but  has  the  power 
of  producing  ions  in  the  gas  with  which  it  is  mixed  by  means 
of  the  rays  which  it  emits. 

122.  Some  Properties  of  Thorium  Emanation. — Some  of  trie 
fundamental  properties  of  the  emanation  may  be  investigated 
by  means  of  the  apparatus  shown  in  Fig.  64.     C  is  a  glass  or 
metal  tube  about  8  or  10  cm.  long  and  3  or  4  cm.  in  diameter. 
Close  the  ends  with  rubber  stoppers  through  which  pass  smaller 
glass  or  metal  tubes  as  shown.     In  the  lower  part  of  C  place 

190 


THORIUM    EMANATION  19! 

a  shallow  trough  containing  a  quantity  of  thorium  oxide.  FH 
is  a  testing  vessel  consisting  of  a  brass  cylinder  about  30  cm. 
long  and  6  cm.  in  diameter.  K  is  a  central  insulated  electrode 
consisting  of  a  brass  rod  20  cm.  long  and  connected  to  an 
electrometer.  Insulate  the  cylinder  FH  and  connect  it  to  a 
battery  in  the  usual  manner.  Connect  the  cylinders  FH  and  C 


*1 


FIG.  64. 


by  a  glass  or  metal  tube  a  few  centimeters  long  between  the 
points  D  and  E.  Pass  a  slow  current  of  air  from  a  gasometer 
through  a  wash  bottle  A  containing  sulphuric  acid  to  dry  it,  and 
a  tightly  packed  plug  of  cotton  wool  in  a  glass  bulb  B  to  remove 
dust  and  spray,  and  thence  through  the  rest  of  the  system. 

As  soon  as  the  current  of  air  is  started  test  the  ionization 
current  in  the  vessel  FH.  In  all  these  measurements  on  the 
ionization  effects  of  the  emanations  use  the  steady  deflection 
method  of  electrometer  measurement  described  in  §89.  If  the 
current  of  air  is  maintained  steady  the  ionization  in  FH  will 
be  found  to  increase  during  the  first  few  minutes  but  will  soon 
reach  a  steady  value  and  remain  constant  as  long  as  the  air 
current  is  maintained.  Replace  the  tube  between  D  and  E  by 
another  one  containing  a  quantity  of  glass  wool  and  repeat  the 
experiment.  Note  that  the  ionization  in  FH  is  practically  the 
same  as  before.  Again  replace  the  tube  containing  the  glass 
wool  by  a  wash  bottle  containing  water,  so  that  the  current  of 
air  and  emanation  will  have  to  bubble  through  the  water  on  the 
way  from  C  to  FH,  and  again  test  the  ionization  in  FH.  Prac- 
tically the  same  value  for  the  ionization  should  be  found  as 
before. 


I92  EMANATIONS 

Once  more  replace  the  wash  bottle  between  D  and  E  by  an 
insulated  metal  tube  20  or  25  cm.  long  and  2  cm.  in  diameter, 
containing  an  insulated  electrode  along  the  axis  of  the  tube 
similar  to  the  one  described  in  §48.  Establish  a  strong  elec- 
tric field  betwen  this  electrode  and  the  tube  and  again  repeat 
the  ionization  tests  in  FH.  Note  that  the  ionization  current  in 
FH  remains  practically  the  same  as  before. 

These  experiments  clearly  show  that  the  gas  in  the  tube  C 
which  is  carried  into  FH  possesses  some  property  in  addition 
to  being  simply  ionized,  for  if  we  compare  these  experiments 
and  their  results  with  similar  ones  performed  with  uranium 
(§85)  and  with  Rontgen  rays  (§§47,  48)  a  very  essential 
difference  is  presented.  In  the  case  of  the  gas  being  merely 
ionized  by  Rontgen  rays  or  rays  from  uranium  the  ions  were 
all  removed  by  passing  through  wool  or  water  or  an  electric 
field  and  no  ionization  appeared  in  the  testing  vessel  beyond, 
but  in  the  present  instance  the  introduction  of  the  glass  wool 
or  water  or  strong  electric  field  produced  practically  no  effect. 
It  cannot  therefore  be  merely  ions  which  are  conveyed  along 
with  the  air,  for  they  would  be  removed  by  these  agents,  but 
it  must  be  something  which  is  capable  of  producing  ions  after 
it  reaches  the  vessel  FH.  The  emanation  mixed  with  the  air 
behaves  just  like  an  ordinary  gas  in  passing  through  glass 
wool  or  a  liquid  and  it  is  not  charged  as  ions  are,  for  it  is 
unaffected  on  passing  through  an  electric  field.  Many  other 
experiments  confirm  this  gaseous  nature  of  the  emanation. 

123.  Diffusion  of  Thorium  Emanation  through  Solids. — In 
a  lead  plate  about  6  mm.  thick  cut  a  shallow  depression  2  or  3 
mm.  deep  and  6  cm.  square.  Fill  this  with  thorium  oxide  and 
cover  it  with  two  or  three  thicknesses  of  ordinary  foolscap 
paper  and  carefully  wax  down  the  edges  to  prevent  any  escape 
around  the  edges.  Place  this  in  a  closed  testing  vessel  of  the 
form  shown  in  Fig.  50,  which  should  be  perfectly  free  from 
any  air  currents.  Allow  it  to  remain  for  ten  minutes  or  so 
and  then  test  the  ionization  current  between  the  plates.  The 
strong  ionization  produced  cannot  be  due  to  the  ordinary  a 
rays  of  the  thorium,  for  they  will  be  practically  all  cut  off  by 


THORIUM    EMANATION  193 

the  paper  and  the  ionization  will  be  found  too  strong  to  be 
accounted  for  by  the  ft  or  y  rays.  The  emanation  must  there- 
fore diffuse  through  the  paper  and  ionize  the  gas  above  it. 

Repeat  this  test,  using,  instead  of  paper,  a  sheet  of  alumin- 
ium foil  about  .002  cm.  thick.  Quite  a  large  quantity  of  ema- 
nation will  be  found  to  penetrate  even  this  thickness,  which 
would  be  sufficient  to  cut  off  practically  all  the  a  rays  from 
thorium.  Other  substances  in  very  thin  sheets  such  as  card- 
board, mica,  etc.,  might  also  be  tried.  The  emanation  will  be 
found  capable  of  diffusing  through  a  number  of  different  sub- 
stances just  as  an  ordinary  gas  would  do. 

124.  Nature  of  Radiations  Emitted  by  Thorium  Emana- 
tion.— Since  the  emanation  is  distributed  throughout  the  gas 
surrounding  the  thorium  the  radiations  emitted  by  the  emana- 
tion arise  from  all  points  throughout  the  volume  of  the  gas. 
In  order  to  examine  the  nature  of  these  radiations  experi- 
mentally special  methods  must  be  used  in  order  to  confine  the 
emanation  within  a  definite  space  and  to  ionize  a  definite 
volume  of  air  free  from  the  emanation.  This  may  be  done 
by  the  following  method  due  to  Rutherford. 

AB  is  a  lead  box  (Fig.  65)  about  15  cm.  square  and  I  cm. 
deep.  In  the  top  of  the  box  cut  a  hole  about  7  cm.  square 


and  cover  it  with  very  thin  mica  not  more  than  about  .0015  cm. 
thick  and  carefully  wax  down  the  edges.  Before  closing  the 
box  wrap  a  quantity  of  thorium  oxide  in  paper  and  place  it  in 
the  box  in  the  position  marked  T  remote  from  the  mica 
window,  so  that  it  may  be  shielded  by  the  lead  and  so  that 


194  EMANATIONS 

the  direct  radiations  from  it  may  not  fall  upon  the  mica.  The 
remaining  space  in  the  lead  box  will  soon  become  filled  with 
emanation  after  diffusing  through  the  paper.  The  radiation 
emitted  by  the  emanation  will  then  penetrate  the  mica  and 
ionize  the  air  between  the  cover  D  of  the  box  and  the  plate  C. 
The  ionization  produced  between  the  plates  by  these  rays  may 
then  be  examined  in  the  usual  manner. 

Measure  the  ionization  between  the  plates;  then  cover  the 
mica  with  a  sheet  of  the  very  thin  aluminium  foil  used  in  the 
absorption  experiments  (§90)  and  measure  the  ionization 
again.  Repeat  this,  adding  a  sheet  at  a  time  until  the  radia- 
tion is  practically  all  absorbed.  It  will  be  found  that  it  re- 
quires a  thickness  of  only  about  .0015  cm.  of  aluminium  to 
completely  absorb  all  the  rays  which  must  therefore  be  of  the 
a  ray  type.  Since  this  thickness  of  aluminium  absorbs  all 
the  *  rays  given  off  by  the  emanation  there  can  be  no  (3  or 
y  rays,  for  they  would  penetrate  a  much  greater  thickness  even 
after  passing  through  the  mica  window. 

These  tests  should  be  made  as  soon  as  possible  after  the 
introduction  of  the  thorium  compound  into  the  lead  box,  for 
if  it  be  allowed  to  stand  a  few  hours  the  excited  activity,  which 
will  be  described  in  the  next  chapter,  will  begin  to  make  its 
appearance. 

125.  Decay  of  Thorium  Emanation. — The  activity  of  any 
given  quantity  of  the  emanation  is  not  permanent,  but  rapidly 
decays  with  the  time  if  the  supply  of  emanation  is  not  con- 
tinuously renewed.  This  may  be  shown  experimentally  by 
means  of  the  apparatus  indicated  in  Fig.  64.  Wrap  a  quantity 
of  thorium  oxide  in  a  piece  of  ordinary  foolscap  paper  which 
will  absorb  most  of  the  ordinary  radiation  from  the  oxide  but 
will  allow  the  emanation  to  pass  through.  Pass  a  slow  current 
of  air,  freed  from  dust  and  moisture,  through  the  system  for 
about  ten  minutes  until  the  saturation  ionization  current  in 
the  vessel  FH  reaches  a  steady  value.  Measure  this  current 
by  the  steady  deflection  method.  Then  stop  the  air  current  and 
measure  the  saturation  current  at  intervals  of  about  twenty  or 
twenty-five  seconds  for  a  period  of  about  ten  minutes.  Ob- 


THORIUM    EMANATION  195 

serve  that  this  current  gradually  decreases  with  increase  of 
time  until  it  finally  reaches  a  zero  value.  Plot  a  curve  with 
current  values  for  ordinates  and  time  for  abscissae,  taking  the 
maximum  value  of  the  ionization  current  as  unity.  The  curve 
will  be  seen  to  be  of  the  same  general  form  as  the  decay  curve 
for  Ur.  X  or  Th.  X,  with  the  exception  of  the  slight  initial 
irregularity  in  the  Th.  X  curve.  The  activity  of  the  emana- 
tion therefore  decays  according  to  an  exponential  law.  This 
law  may  be  represented  as  in  previous  cases  by  the  equation 
It  =  70€~x*,  where  70  represents  the  maximum  activity,  while  It 
represents  the  activity  after  a  time  t  and  A  is  a  constant.  If  the 
measurements  are  carefully  made  the  period  of  decay,  that  is 
the  time  taken  for  the  activity  to  fall  to  half  its  maximum 
value,  should  be  fifty-four  seconds. 

As  has  been  previously  noted,  the  particles  of  emanation  are 
not  charged,  and  therefore  this  decay  cannot  be  due  to  any 
removal  of  the  particles  of  emanation  by  the  electric  field. 
This  may  be  tested  by  applying  different  electric  fields  and  the 
rate  of  decay  will  be  found  to  be  unaffected.  Nor  can  it  be 
a  case  of  recombination  of  any  sort,  since  the  particles  are 
uncharged,  and  also  the  rate  of  decay  is  entirely  different  from 
the  rate  of  recombination  of  ions.  The  loss  of  activity  must 
be  due  to  a  gradual  change  or  transformation  of  the  emana- 
tion into  some  other  substance  according  to  the  theory  of 
successive  changes. 

126.  Increase  of  Current  with  Time. — Place  a  quantity  of 
thorium  oxide  covered  with  a  sheet  of  paper  in  a  closed  testing 
vessel  of  the  form  shown  in  Fig.  50.  As  soon  as  possible  after 
the  introduction  of  the  oxide,  measure,  by  the  steady  deflection 
method,  the  saturation  current  and  repeat  the  measurement 
at  intervals  of  twenty  or  twenty-five  seconds  for  a  period  of 
ten  or  fifteen  minutes.  The  current  will  be  found  to  increase 
with  time  until  it  reaches  a  constant  maximum.  Plot  the  usual 
current-time  curve  and  it  will  be  found  to  be  complementary  to 
the  decay  curve  just  studied  in  the  last  paragraph,  showing  that 
the  activity  gradually  rises  to  a  steady  maximum  and  the  rate 
of  increase  is  the  same  as  the  rate  of  decay  of  the  emanation. 


196  EMANATIONS 

This  curve  follows  an  exponential  law  similar  to  the  recovery 
curves  for  uranium  and  thorium  and,  using  the  usual  notation, 
may  be  expressed  by  the  equation  /*  =  /0(i — 6~x*).  This 
curve  and  the  decay  curve  for  the  emanation  bear  the  same 
relation  to  each  other  that  the  recovery  curve  of  thorium  bears 
to  the  decay  curve  of  thorium  X. 

What  is  the  significance  of  this  rise  of  activity?  The 
thorium  is  continually  emitting  emanation,  and  when  the 
thorium  is  introduced  into  the  testing  vessel  the  amount  of 
emanation  gradually  increases,  as  it  is  not  allowed  to  escape 
from  the  vessel.  If  the  emanation  did  not  decay  it  would 
continue  to  increase  indefinitely,  but  since  it  decays  with  time 
the  increase  continues  until  the  amount  produced  per  second  is 
equal  to  the  amount  which  disappears  per  second,  and  there- 
fore the  activity  gradually  rises  until  an  equilibrium  state  is 
reached  and  the  activity  then  remains  constant  as  long  as  a 
constant  supply  of  emanation  is  available.  The  period  of  rise 
of  the  activity,  that  is  the  time  required  to  rise  to  half  its 
maximum  value,  is  equal  to  the  period  of  decay,  namely,  fifty- 
four  seconds. 

127.  Radium  Emanation. — Not  long  after  the  discovery  of 
thorium  emanation  it  was  shown  that  radium  compounds  also 
give  rise  to  an  emanation  possessing  properties  very  similar 
to  the  thorium  emanation.  One  respect  in  which  these  two 
emanations  differ  markedly  from  each  other  is  in  the  rate  of 
decay.  The  emanation  from  radium  takes  a  much  longer 
time  to  decay  than  that  from  thorium.  Somewhat  special 
methods  must  be  used  in  testing  this  decay  of  radium  emana- 
tion, owing  to  the  formation  of  what  is  known  as  excited 
activity,  which  will  be  discussed  in  the  following  chapter.  If 
a  quantity  of  emanation  were  introduced  into  a  testing  vessel 
and  allowed  to  remain  and  the  activity  measured  by  the  method 
used  in  the  case  of  thorium  this  excited  activity  which  gives 
out  radiations  would  be  formed  so  quickly  that  the  result  would 
be  complicated.  The  effect  due  to  this  excited  activity  may 
be  eliminated  by  the  following  method. 

Radium  chloride  dissolved  in  water  gives  off  much  larger 


RADIUM    EMANATION  197 

quantities  of  emanation  than  in  the  solid  state.  Slowly  bubble 
air  through  such  a  solution  and  collect  the  air  mixed  with 
emanation  in  a  gas  holder  over  mercury.  As  soon  as  the  gas 
holder  is  filled  draw  off  a  measured  amount  of  this  air  mixed 
with  emanation  in  a  gas  pipette  and  introduce  this  meas- 
ured quantity  into  a  testing  vessel  of  a  form  similar  to  the 
one  shown  in  Fig.  64,  which  must  be  air-tight.  Measure  imme- 
diately the  ionization  current  and  then  blow  the  air  and  emana- 
tion out  of  the  vessel  so  that  it  may  not  remain  in  the  vessel 
any  length  of  time.  At  intervals  of  twelve  or  fifteen  hours  for 
a  period  of  about  twenty-five  or  thirty  days  repeat  this  opera- 
tion, drawing  off  the  same  measured  amount  of  air  and  emana- 
tion, introducing  it  into  the  testing  vessel  and  measuring  the 
current.  Plot  the  usual  current-time  curve  and  note  its  simi- 
larity to  the  corresponding  curve  for  thorium  emanation.  In 
this  instance,  however,  the  period  of  decay  is  very  much  longer, 
being  3.75  days  instead  of  fifty-four  seconds.  This  difference 
in  period  very  clearly  differentiates  these  two  emanations. 
The  activity  of  the  radium  emanation  is  due  to  the  emission  of 
a  rays  just  as  in  the  case  of  thorium  emanation. 

128.  Rise  of  Activity  of  Radium. — Dissolve  a  small  quan- 
tity of  radium  chloride  in  water  and  bubble  a  current  of  air 
through  the  solution  for  a  few  hours,  so  as  to  remove  the  ema- 
nation from  it.     Then  evaporate  the  solution  to  dryness  and 
test  the  activity  of  the  residue.     Test  this  activity  at  intervals 
of  twelve  or  fifteen  hours  and  observe  the  gradual  increase 
with  time.     Continue  these  tests  over  a  period  of  about  twenty 
or  twenty-five   days   and  plot  the  usual   current-time   curve. 
This  curve  should  be  found  to  be  complementary  to  the  decay 
curve,  as  in  the  case  of  thorium  and  the  rate  of  recovery  of 
activity  equal  to  the  rate  of  decay  of  the  emanation.     This, 
of  course,  is  due  to  the  gradual  production  of  emanation,  which 
accumulates  until  an  equilibrium  state  is  reached,  when  the 
rate  of  production  is  equal  to  the  rate  of  decay. 

129.  Actinium  Emanation. — Actinium  compounds  also  give 
rise  to  an  emanation  possessing  properties  similar  to  the  other 
emanations.     The  most  distinguishing  characteristic  of  it  is  its 


198  EMANATIONS 

period  of  decay,  which  is  extremely  short  and  consequently 
somewhat  difficult  to  measure.  It  decays  to  half  value  in  the 
short  period  of  3.7  seconds.  Like  the  other  emanations,  its 
activity  is  due  to  the  emission  of  a  rays. 

130.  Effect  of  Conditions  on  Emanating  Power. — The  dif- 
ferent compounds  of  thorium  vary  greatly  in  the  amount  of 
emanation  given  off  under  ordinary  conditions.  Although  the 
percentage  of  thorium  present  in  a  given  weight  of  the  com- 
pound may  not  be  very  different  in  the  various  compounds,  yet 
the  amount  of  emanation  given  off  by  equal  weights  of  the 
different  compounds  varies  enormously.  For  instance,  thor- 
ium nitrate  in  the  solid  form  emits  only  about  %0o  as  much 
emanation  as  the  same  weight  of  thorium  hydroxide.  Even 
different  preparations  of  the  same  compound  vary  somewhat 
among  themselves.  The  oxide  of  thorium  is  one  of  the  most 
powerfully  emanating  compounds  of  thorium.  The  different 
compounds  of  radium  also  show  differences  in  emanating 
power. 

The  amount  of  emanation  given  off  is,  however,  independent 
of  the  nature  of  the  gas  surrounding  the  emanating  body.  If 
the  ionization  current  due  to  the  rays  from  the  emanation  be 
measured  in  different  gases  it  will  of  course  vary  with  the 
nature  of  the  gas,  but  this  is  due  to  the  different  amount  of 
ionization  produced  in  different  gases  by  the  same  rays,  and 
not  to  any  difference  in  the  amount  of  emanation  present. 

The  rate  of  emission  of  emanation  is  also  independent  of  the 
pressure  of  the  gas.  Wrap  a  quantity  of  thorium  oxide  in 
paper  to  absorb  the  a  rays  given  out  by  the  thorium  itself  and 
place  it  in  an  air-tight  vessel  of  the  form  represented  in  Fig. 
50.  Measure  the  saturation  current  at  different  pressures  and 
it  will  be  found  to  vary  directly  as  the  pressure.  But  the 
ionization  produced  by  any  radio-active  source  is  proportional 
to  the  pressure  and  therefore  the  source  in  this  instance,  which 
is  the  emanation,  must  be  independent  of  the  pressure  of 
the  gas. 

The  emanating  power  of  a  compound  of  thorium  or  radium 
depends  upon  the  state  of  moisture  of  the  gas  surrounding  the 


EMANATING  POWER  199 

compound.  The  emanating  power  is  greater  in  a  moist  gas 
than  in  a  dry  one.  The  introduction  of  moisture  into  the  sur- 
rounding gas  will  increase  by  several  times  the  amount  of 
emanation  given  off. 

If  the  emanating  compound  be  placed  in  solution  the  emanat- 
ing power  is  enormously  increased.  In  some  cases  it  will  be 
increased  several  hundred  times  by  simply  dissolving  the  ra- 
dium or  thorium  compound  in  water.  Experiments  show  that 
this  is  really  not  due  to  a  difference  in  the  rate  of  production 
in  the  solid  form  and  in  the  dissolved  state,  but  that  in  the 
solid  form  the  emanation  when  produced  is  occluded  in  the 
solid  and  not  allowed  to  escape  so  rapidly,  while  in  the  solution 
it  escapes  much  more  easily.  Therefore,  to  obtain  large  quan- 
tities of  emanation  it  is  always  advantageous  to  place  the 
emanating  compound  in  solution. 

Temperature  is  a  very  important  factor  in  connection  with 
the  emanating  power  of  different  compounds.  If  ordinary 
thoria,  for  instance,  be  placed  in  a  platinum  tube  and  gradually 
heated  up  to  a  dull  red  heat  the  rate  of  emission  of  emanation 
will  gradually  increase  to  several  times  its  original  value  and 
will  continue  to  escape  at  that  rate  if  the  temperature  is  main- 
tained constant.  When  the  temperature  is  lowered  again  the 
rate  of  emission  of  emanation  will  return  to  its  original  value. 
Also  if  the  temperature  is  reduced  to  the  neighborhood  of 
—  80°  C.  the  emanating  power  is  reduced  to  only  a  small  frac- 
tion of  its  value  at  ordinary  temperatures,  but  is  restored  again 
when  the  temperature  returns  to  its  ordinary  value. 

If  the  thoria,  for  instance,  in  the  platinum  tube  be  heated  to 
a  white  heat,  a  remarkable  change  takes  place.  At  this  high 
temperature  the  emanating  power  is  greatly  decreased  and  on 
cooling  the  thoria  does  not  regain  its  emanating  power,  but  it 
is  permanently  reduced  to  only  a  small  fraction  of  its  original 
value.  This  de-emanating,  as  it  is  called,  of  the  compound  is 
permanent,  but  its  original  power  of  emitting  emanation  may 
be  restored  by  dissolving  the  compound  and  then  separating  it 
from  the  solution.  Radium  compounds  may  also  be  de-emanated 
and  afterwards  restored  by  a  similar  process. 


200 


EMANATIONS 


131.  Condensation  of  the  Emanations. — These  emanations 
act  in  all  respects  like  gases.  One  of  the  most  conclusive 
proofs  of  their  gaseous  nature  lies  in  the  fact  that  they  may  be 
condensed  by  very  low  temperatures.  Rutherford  and  Soddy 
were  the  first  to  show  this  experimentally  and  used  the  fol- 
lowing method :  The  radium  emanation  is  stored  in  a  reservoir 
R  (Fig.  66)  and  may  be  forced  out  of  the  reservoir  by  raising 


FIG.  66. 

the  level  of  the  liquid.  This  is  connected  through  the  stop- 
cock S  to  the  horizontal  tube  above  through  which  a  steady 
stream  of  gas  may  be  sent  from  a  gasometer  and  on  its  way 
passes  through  a  bulb  B  containing  drying  material.  CDE 
is  a  spiral  made  from  a  copper  tube  of  a  total  length  of  310 
cm.  and  internal  diameter  of  2  mm.  FH  is  a  cylindrical  test- 
ing vessel  of  the  usual  form  containing  an  insulated  electrode 
K,  so  that  by  connecting  this  electrode  to  the  electrometer  and 
the  cylinder  to  a  battery  any  ionization  current  produced  in  the 
cylinder  may  be  measured.  If  the  stopcock  S  be  opened  and 
the  liquid  in  R  be  raised  while  a  slow  stream  of  gas  is  passed 
along  the  horizontal  tube  the  emanation  will  be  carried 
through  the  spiral  D  and  into  FH,  where  an  ionization 
current  will  be  produced.  If  the  copper  spiral  D  be  im- 
mersed in  liquid  air  the  emanation  will  not  reach  the  ves- 


CONDENSATION   OF   EMANATIONS  2OI 

sel  FH,  as  will  be  indicated  by  the  fact  that,  although  the 
current  of  gas  is  still  flowing,  the  ionization  current  in  FH 
ceases.  If  while  the  spiral  D  is  immersed  in  the  liquid  air  the 
stopcock  5*  be  closed  so  as  to  shut  off  the  supply  of  emanation 
and  then  the  spiral  be  removed  from  the  liquid  air  while  the 
stream  of  air  through  the  system  is  still  flowing,  there  will 
be,  shortly  after  removal,  a  sudden  movement  of  the  electrom- 
eter needle  indicating  a  sudden  production  of  ions  in  FH. 
These  experiments  show  that  when  the  spiral  tube  is  immersed 
in  the  liquid  air  the  emanation  which  previously  passed  through 
along  with  the  air  is  condensed  and  remains  in  the  spiral,  but 
when  removed  from  the  liquid  air  and  its  temperature  allowed 
to  rise  the  emanation  volatilizes  and  is  then  carried  over  into 
FH  and  manifests  itself  as  usual  by  producing  ions. 

The  temperature  at  which  the  emanation  volatilizes  was 
determined  in  the  following  manner:  A  current  of  about  0.9 
ampere  from  a  storage  battery  was  sent  through  the  spiral  by 
leads  soldered  to  the  points  a  and  b  and  this  current  was  meas- 
ured by  a  Weston  ammeter  M.  Two  potential  leads  were 
soldered  to  the  points  c  and  d  and  the  potential  read  by  a  milli- 
voltmeter  N.  Any  change  of  resistance  in  the  spiral  due  to 
change  of  temperature  could  be  determined  from  the  current 
and  drop  of  potential.  The  copper  spiral  was  thus  used  as  a 
resistance  thermometer,  and  was  carefully  calibrated  by  meas- 
uring its  resistance  at  the  known  temperatures  of  the  boiling 
and  freezing  points  of  liquid  ethylene  and  the  boiling  point  of 
liquid  air.  After  calibrating  the  spiral  a  quantity  of  emanation 
was  condensed  in  it  by  immersing  it  first  in  a  liquid  ethylene 
bath  and  then  still  further  cooling  this  bath  by  liquid  air.  The 
temperature  was  then  allowed  to  rise  slowly,  and  when  the 
emanation  made  its  appearance  in  the  testing  vessel  FH  the 
temperature  of  the  spiral  was  determined.  It  was  found  that 
the  radium  emanation  volatilized  at  — 150°  C. 

Thorium  emanation,  on  account  of  its  very  rapid  rate  of 
decay,  presents  considerable  difficulty  in  the  determination  of 
the  temperature  at  which  it  condenses  or  volatilizes.  In  the 
time  required  to  make  observations  a  very  large  proportion  of 
its  activity  will  have  decayed  and  consequently  it  is  extremely 


202  EMANATIONS 

difficult  to  make  definite  measurements.  By  using  special 
methods  however  Rutherford  and  Soddy  found  that  the  thorium 
emanation  began  to  condense  at  — 120°  C.,  but  that  some  of 
it  might  escape  condensation  even  as  low  as  — 150°  C. 

132.  Decay  of  Emanation  at  Low  Temperatures. — The  rate 
of  decay  of  the  activity  of  these  emanations  at  the  temperature 
of  liquid  air,  that  is,  while  they  are  condensed,  has  also  been 
determined,  and  it  is  found  that  the  rate  of  decay  is  unaltered 
at  this  very  low  temperature,  and  even  the  fact  of  the  ema- 
nations being  condensed  does  not  affect  the  rate  of  decay.     The 
activity  of  the  emanation  in  the  condensed  form  decays  at  just 
the  same  rate  as  in  the  gaseous  form. 

This  may  be  easily  tested  in  the  case  of  radium  emanation. 
Pass  a  known  quantity  of  emanation  through  the  system  (Fig. 
66)  into  the  testing  vessel  at  ordinary  temperatures  and  meas- 
ure its  activity.  Then  condense  an  equal  quantity  in  the  spiral 
and  leave  it  in  the  condensed  state  for  a  known  interval  and 
then  after  allowing  it  to  volatilize  pass  it  into  the  testing  vessel 
and  measure  its  activity.  Repeat  this  at  increasing  intervals 
and  determine  the  regular  decay  curve.  For  this  purpose  the 
emanation  should  be  derived  from  a  constant  source,  that  is, 
from  a  vessel  containing  a  solution  of  a  radium  compound 
which  is  continually  giving  off  fresh  emanation,  for  if  the 
emanation  be  stored  in  a  separate  vessel  its  activity  will  of 
course  decay -and  the  source  of  activity  will  not  be  constant. 
Similar  results  may  be  obtained  with  thorium  emanation,  but 
the  experiments  have  to  be  made  very  rapidly  and  by  special 
methods  as  the  rate  of  decay  is  so  extremely  rapid. 

133.  Phosphorescent  Action  of  the  Emanations. — The  ema- 
nations show  marked  phosphorescent  phenomena  similar  to  the 
phosphorescent  action  of  other  radio-active  bodies.     This  may 
be  shown  by  placing  a  small  quantity  of  moist  radium  bromide 
on  a  zinc  sulphide  screen.     The  luminosity  will  spread  over  the 
surface  of  the  screen  as  the  emanation  diffuses  around  and 
the  luminosity  can  be  made  to  move  about  by  gently  blowing  on 
the  screen.     If  the  moist  radium  compound  be  placed  in  a  glass 
tube  and  a  stream  of  air  passed  through  the  tube  a  zinc  sul- 
phide screen  placed  at  the  mouth  of  the  tube  will  be  brightly 


PHOSPHORESCENT   ACTION    OF    EMANATIONS  2O3 

illuminated  by  the  emanation.  A  variety  of  materials  show 
this  phosphorescent  action  when  the  emanation  comes  in  con- 
tact with  them. 

This  action  is  also  quite  marked  at  even  the  low  temperature 
of  liquid  air,  which  may  be  demonstrated  in  a  very  interesting 
manner.  Place  a  quantity  of  willemite  crystals  in  a  glass  U 
tube  of  about  8  mm.  diameter.  Connect  this  to  a  reservoir 
containing  a  supply  of  radium  emanation.  Immerse  the  U 
tube  in  liquid  air  and  pass  a  quantity  of  emanation  through  the 
tube.  The  emanation  by  being  condensed  will  be  left  behind 
in  the  tube.  If  the  tube  be  removed  from  the  liquid  air  it  will 
be  found  that  the  crystals  nearest  the  end  where  the  emanation 
entered  the  tube  show  luminosity  while  those  at  the  other  end 
do  not.  If  the  ends  of  the  tube  be  closed  and  the  temperature 
allowed  to  rise  this  luminosity  will  gradually  diffuse  throughout 
all  the  contained  crystals.  This  indicates  that  the  emanation 
by  being  condensed  near  the  beginning  of  the  crystals  does 
not  reach  those  farther  along  the  tube,  and  produces  luminos- 
ity at  this  point  even  at  this  low  temperature,  but  when  the 
temperature  rises  the  emanation  volatilizes  and  is  able  to 
diffuse  throughout*  the  tube  and  produces  phosphorescence 
in  the  remaining  crystals.  The  movement  of  the  emanation 
through  the  tube  can  thus  be  followed  by  the  eye. 

After  the  volatilization  takes  place  and  the  tube  is  filled  with 
the  emanation  it  may  be  partially  concentrated  at  any  point  by 
applying  to  the  outside  of  the  U  tube  a  piece  of  cotton  wool 
saturated  with  liquid  air.  The  tube  is  thus  cooled  locally  by 
the  liquid  air  and  the  emanation  condenses  and  is  concen- 
trated at  that  point. 

134.  Source  of  the  Emanations. — In  a  thorium  compound  in. 
equilibrium  both  thorium  and  Th.  X  are  present.  Which 
of  these  is  the  direct  source  which  gives  rise  to  the  emana- 
tion? If  the  thorium  X  be  separated  in  the  usual  way 
from  the  thorium  compound  the  thorium  residue  even  in 
solution  will  at  first  emit  no  emanation,  but  the  solution 
containing  the  Th.  X  will  possess  unusual  emanating  power. 
Not  only  so,  but  the  thorium  residue  will  gradually  regain 
its  emanating  power,  while  the  Th.  X  gradually  loses  its 


204  EMANATIONS 

power  of  emitting  emanation.  The  rate  at  which  the  tho- 
rium regains  its  emanating  power  is  the  same  as  the  rate 
of  decay  of  the  emanating  power  of  the  Th.  X.  In  addition, 
these  rates  of  recovery  and  decay  of  emanating  power  are 
exactly  the  same  as  the  rates  of  recovery  and  decay  of  the 
radio-activity  of  thorium  and  Th.  X  respectively.  These  re- 
sults show  then  that,  since  no  emanation  is  produced  just  after 
the  Th.  X  is  removed  and  the  Th.  X  itself  possesses  strong 
emanating  power,  and  besides,  since  the  rise  of  activity  of 
thorium  residue  is  due  to  the  production  of  Th.  X  and  also  the 
thorium  regains  emanating  power  at  the  same  rate  that  its 
activity  is  regained,  that  is  at  the  same  rate  that  Th.  X  is 
produced,  the  emanation  must  arise  directly  from  the  Th.  X 
and  not  from  the  thorium  itself.  Since  the  activity  of  the 
Th.  X  and  its  emanating  power  decay  at  the  same  rate  the 
activity  of  Th.  X  is  therefore  proportional  to  its  emanating 
power.  According  to  the  theory  of  successive  changes  then 
the  production  of  emanation  must  be  a  result  of  the  decay  of 
Th.  X,  that  is,  the  Th.  X  must  be  gradually  changing  spon- 
taneously into  the  emanation.  Th.  X  and  the  emanation  have 
distinct  chemical  and  physical  properties  and  have  quite  dis- 
tinct rates  of  decay,  one  being  3.7  days  and  the  other  54 
seconds.  They  are  therefore  entirely  distinct  substances. 

Similar  remarks  apply  to  Act.  X  and  actinium  emanation. 
The  latter  arises  directly  from  the  Act.  X. 

There  is  a  difference  in  the  case  of  radium  emanation,  for 
no  substance  has  as  yet  been  discovered  in  connection  with 
radium  corresponding  to  Th.  X  or  Act.  X.  As  far  as  is  known 
there  is  no  such  product  as  radium  X.  Radium  emanation, 
unlike  the  other  two,  arises  directly  from  the  radium  itself 
without  any  other  product  intervening.  According  to  the  same 
theory  the  radium  is  continuously  changing  into  emanation, 
but  as  will  be  seen  later  the  rate  of  change  is  extremely  slow. 

Since  these  emanations  are  decaying  at  a  more  or  less  rapid 
rate  they  must,  on  following  out  this  theory  of  successive 
changes,  be  going  through  a  similar  process  of  transformation 
into  some  other  product.  This  will  be  seen  in  the  following 
chapter  to  be  the  case. 


CHAPTER  XIV. 


EXCITED  ACTIVITY. 

135.  Active  Deposit. — If  a  solid  body  be  exposed  in  a  closed 
vessel  to  the  emanation  from  thorium,  radium  or  actinium 
its  surface  becomes  coated  with  an  extremely  thin,  solid  deposit 
of  material  which  is  intensely  radio-active.  This  active  de- 
posit, as  it  is  called,  is  a  very  extraordinary  substance  and 
possesses  remarkable  properties,  the  study  of  which  has  thrown 
an  immense  amount  of  light  on  the  processes  going  on  in  radio- 
active bodies. 

In  an  air-tight  vessel,  Fig.  67,  about  25  cm.  square  place  a 
heavy  lead  block  C  about  8  cm.  square  and  12  cm.  high.  In 
the  top  of  this  cut  a  recess  into 
which  a  small  beaker  D  may  fit. 
In  this  beaker  put  a  solution 
of  either  a  thorium  or  radium 
compound.  At  different  posi- 
tions, such  as  E,  F  and  H,  place 
plates  of  metal  about  3  or  4 
cm.  square  and  close  the  vessel 
and  leave  them  for  six  or  eight 
hours.  At  the  end  of  this  time 
remove  each  of  the  plates  separately  and  placing  them  in 
a  testing  vessel  of  the  form  in  Fig.  50,  test  them  for  radio- 
activity. Observe  that  they  are  not  only  radio-active,  but 
that  the  plates  from  the  different  positions  in  the  vessel 
possess  practically  the  same  amount  of  radio-activity  when 
tested  immediately  after  removal  from  the  vessel  AB.  These 
plates  have  thus  acquired  radio-active  properties  while  in  the 
vessel.  This  acquired  property  cannot  be  a  result  of  the  direct 
radiations  given  off  by  the  radio-active  body  in  D,  for  the 
plate  H,  which  was  entirely  shielded  from  these  rays  by  the 
lead  block,  is  just  as  active  as  E,  which  was  in  the  direct  line 

205 


FIG.  67. 


206 


EXCITED   ACTIVITY 


of  the  rays.  It  must  be  due  to  something  which  is  distributed 
equally  throughout  the  vessel  and  the  only  radio-active  material 
which  is  thus  equally  distributed  is  the  emanation. 

Make  a  similar  test  for  plates  made  of  other  materials,  both 
conductors  and  non-conductors.  The  amount  of  activity  ac- 
quired in  the  same  time  should  be  practically  equal  in  all  cases. 
This  acquired  radio-activity  therefore  does  not  depend  upon 
the  material  on  which  it  is  deposited. 

136.  Concentration  on  Negative  Electrode. — In  an  air-tight 
metal  vessel  AB,  Fig.  68,  place  a  quantity  of  powdered  tho- 
rium oxide  or  a  solution  of  radium  bromide  contained  in  a 
glass  crystallizing  dish.  The  top  of  the  vessel  should  consist 
of  a  plate  which  is  removable.  In  the  top  of  this  plate  insert 
a  short  metal  tube  ab  about  2  cm.  in  diameter.  Make  an 
ebonite  stopper  to  fit  this  tube  and  pass  a  stout  wire  D  through 


- 


FIG.  68. 

this  ebonite  stopper.  Place  the  wire  in  position  and  close  the 
joints  in  the  cover  with  wax  and  connect  the  wire  to  the 
negative  pole  of  a  battery  of  two  or  three  hundred  volts  while 
the  vessel  AB  is  connected  to  the  positive  pole  of  the  battery. 
Allow  the  system  to  remain  thus  for  several  hours.  Then 
remove  the  ebonite  stopper  and  wire  (closing  the  tube  with 
another  stopper  to  prevent  the  escape  of  emanation)  and  test 


ACTIVE   DEPOSIT  2C>7 

the  activity  of  the  wire  in  the  testing  vessel  (Fig.  50)  and  note 
that  it  is  intensely  radio-active.  Repeat  the  experiment  for 
the  same  length  of  time,  but  in  this  case  connect  the  wire  to 
the  positive  pole  and  the  vessel  to  the  negative  pole  of  the 
battery.  On  testing  the  wire  after  the  same  time  of  exposure 
it  should  be  practically  free  from  any  activity  if  it  has  been 
exposed  to  thorium  emanation,  but  will  possess  a  very  little 
if  radium  emanation  were  used.  Repeat  the  experiment  again 
without  connecting  either  wire  or  vessel  to  the  battery.  On 
testing  the  wire  under  these  conditions  it  should  possess  some 
radio-activity,  but  not  nearly  so  much  as  when  the  wire  was 
charged  negatively.  This  active  deposit  from  whatever  source 
it  is  derived  may  be  concentrated  on  a  negatively  charged 
electrode  but  not  on  a  positively  charged  one,  except  to  a  very 
slight  extent  in  the  case  of  radium.  By  this  method  of  con- 
centration a  thin  wire  may  be  made  several  thousand  times 
more  active  per  unit  area  of  surface  than  the  active  compound 
from  which  it  is  derived. 

This  radio-activity  which  is  thus  produced  in  a  non-active 
body  by  exposure  to  an  active  substance  is  usually  called  ex- 
cited radio-activity.  It  will  be  shown  later  that  this  activity 
arises  from  a  material  deposit  called  the  active  deposit. 

137.  Source  of  Excited  Activity. — It  has  been  noted  inci- 
dentally in  §  135  that  the  emanation  is  the  most  probable  source 
from  which  the  excited  activity  can  arise.  This  conclusion  is 
confirmed  by  a  further  study  of  the  conditions  governing  its 
production. 

In  the  bottom  of  the  apparatus,  Fig.  68,  place  the  thorium 
compound  or  the  radium  solution  in  a  shallow  vessel  which 
may  easily  be  covered  over.  Cover  this  vessel  with  several 
sheets  of  paper  which  will  cut  off  the  a  rays,  but  will  allow  the 
emanation  to  diffuse  through.  Connect  the  central  wire  to  the 
negative  pole  as  before  and  after  a  sufficient  time  test  for  the 
concentrated  activity  on  the  wire.  It  will  be  observed  that 
the  cutting  off  of  the  a.  rays  produces  no  diminution  in  the 
amount  of  active  deposit  on  the  wire.  The  active  deposit  can 
not  arise  then  from  the  a  radiation  emitted  by  the  thorium  or 


208  EXCITED  ACTIVITY 

radium.  Now  cover  the  vessel  with  a  sheet  of  mica  and  care- 
fully wax  it  down  to  make  it  gas-tight,  so  that  no  emanation 
at  all  may  escape,  and  repeat  the  previous  test.  Observe  that 
this  time  no  excited  activity  is  obtained  on  the  wire.  This 
indicates  that  the  emanation  is  essential  to  the  production  of 
the  active  deposit.  This  conclusion  is  further  supported  by 
the  fact  that  uranium  and  polonium,  which  do  not  emit  any 
emanation,  do  not  produce  any  active  deposit  while  radium, 
thorium  and  actinium,  all  of  which  give  out  emanations,  pro- 
duce excited  activity  in  bodies  exposed  to  their  emanations. 
In  addition  to  these  facts  the  amount  of  excited  activity  pro- 
duced is  always  proportional  to  the  amount  of  emanation 
present.  This  fact  may  be  very  easily  shown  in  the  case  of 
radium  emanation,  since  it  has  a  long  period  of  decay.  Store 
a  quantity  of  radium  emanation  mixed  with  air  in  a  reservoir 
and  at  intervals  of  six  or  eight  hours  introduce  a  measured 
quantity  of  emanation  into  the  vessel  of  Fig.  68,  and  concen- 
trate the  active  deposit  on  the  central  wire.  At  each  suc- 
cessive interval  the  amount  of  excited  activity  produced  on  the 
wire  will  be  found  to  be  less  than  the  amount  deposited  during 
the  preceding  interval.  The  rate  at  which  the  emanation 
decays  with  the  time  has  already  been  measured,  and  if  the 
amount  of  active  deposit  be  measured  by  the  current  which  it 
produces,  at  each  succeeding  interval  it  will  be  found  to  be 
proportional  to  the  amount  of  emanation  present  at  the  corre- 
sponding time.  If  the  different  emanating  bodies  be  examined 
it  will  be  found  that  the  amount  of  excited  activity  which  they 
are  capable  of  producing  is  always  proportional  to  their  ema- 
nating power. 

This  accumulation  of  facts  shows  conclusively  that  the  active 
deposit  must  arise  from  the  emanation  in  each  case.  In  the 
light  of  the  theory  of  successive  changes,  as  will  be  shown 
later,  the  emanation  in  decaying  is  really  changing  into  the 
active  deposit  which  forms  a  further  link  in  the  chain  of  suc- 
cessive transformations. 

138.  Decay  and  Rise  of  Excited  Activity  from  Thorium. — 
Expose  a  negatively  charged  wire  in  the  vessel  of  Fig.  68  con- 


EXCITED   ACTIVITY    FROM    THORIUM  209 

taining  thorium  oxide  for  a  period  of  twelve  or  fourteen  hours. 
Then  remove  it  and  place  it  in  the  testing  vessel  of  Fig.  50  and 
test  its  activity  at  intervals  of  about  four  hours  at  the  begin- 
ning and  later  at  somewhat  longer  periods.  Plot  a  time- 
activity  curve.  The  activity  will  be  found  to  decay  as  the  time 
advances  and  the  form  of  the  decay  curve  will  show  that  the 
decline  in  activity  takes  place  according  to  an  exponential  law 
of  an  exactly  similar  nature  to  that  representing  the  law  of 
decay  in  the  other  radio-active  products  already  studied.  The 
time  required  for  the  excited  activity  to  fall  to  half  value  will 
be  observed  to  be  eleven  hours. 

Expose  a  fresh  negatively  charged  wire  to  the  thorium 
emanation  for  an  interval  of  only  one  hour,  and,  removing  it, 
test  its  activity  as  quickly  as  possible.  .Replace  it  as  soon  as 
possible  in  the  emanation  again  and  leave  it  for  another  hour, 
and  remove  and  test  again  as  quickly  as  possible.  Repeat 
this  at  intervals  of  two  or  three  hours  for  a  period  of  about 
twenty  or  thirty  hours,  and  after  that  at  longer  intervals  for 
about  three  days.  Plot  a  time-activity  curve  and  observe  that 
the  activity  gradually  increases  with  the  time  of  exposure  to 
the  emanation  until  it  reaches  a  steady  maximum.  Note  also 
that,  except  for  a  little  irregularity  at  the  beginning,  this  curve 
is  complementary  to  the  decay  curve  obtained  above.  These 
two  curves  bear  to  each  other  a  relation  exactly  similar  to  that 
which  the  decay  and  recovery  curves  for  Ur.  X,  Th.  X  or  the 
emanations,  bear  to  each  other.  It  takes  time  for  the  active 
deposit  to  be  produced  from  the  emanation  and  the  amount 
of  the  deposit,  as  measured  by  the  excited  activity,  increases 
until  the  equilibrium  state  is  reached,  when  the  rate  of  produc- 
tion is  equal  to  the  rate  of  decay. 

In  all  the  measurements  on  the  excited  activity  great  care 
must  be  taken  to  ensure  that  the  air  in  the  vessel  containing 
the  emanation  in  which  the  wire  is  exposed  is  free  from  dust, 
as  dust  particles  cause  the  active  deposit  and  its  activity  to  act 
in  a  very  capricious  manner. 

If  instead  of  exposing  the  charged  wire  to  the  emanation 
for  a  period  of  several  hours  the  interval  of  exposure  be 

15 


210  EXCITED  ACTIVITY 

reduced  to  only  a  few  minutes  a  different  phenomenon  presents 
itself.  Expose  the  negatively  charged  wire  in  the  emanation 
vessel  for  about  thirty-five  or  forty  minutes  and  remove  it  and 
test  its  activity  at  short  intervals  of  twenty  minutes  or  so, 
and  plot  the  usual  time-activity  curve.  It  will  be  found  that 
the  activity,  instead  of  beginning  to  decay  after  removal,  at 
first  increases  in  a  marked  degree  for  three  or  four  hours 
until  it  reaches  a  maximum,  and  after  that  decays  according  to 
the  ordinary  exponential  law,  decaying  to  half  its  maximum 
value  in  the  same  time  as  before,  namely  eleven  hours.  If  the 
time  of  exposure  be  made  a  little  longer  this  initial  rise  after 
removal  will  not  be  so  marked,  and  the  longer  the  exposure 
the  less  marked  will  this  initial  rise  in  activity  be,  until  for  a 
long  exposure,  as  already  seen,  the  decay  begins  immediately 
after  removal  and  no  initial  rise  is  observed. 

139.  Explanation  of  the  Decay  Curves  of  the  Active  Deposit 
from  Thorium. — The  marked  difference  between  the  decay 
curves  for  a  short  and  a  long  exposure  indicates  that  the 
process  of  transformation,  according  to  the  general  theory  of 
successive  changes,  is  in  this  instance  somewhat  more  compli- 
cated than  in  the  cases  previously  studied.  Since,  in  the  case 
of  the  short-exposure,  the  activity  at  first  increases,  there  must 
be  a  production  of  active  matter  after  the  removal  from  the 
influence  of  the  emanation.  It  appears  as  though  there  is  at 
first  a  change  taking  place  from  either  a  non-active  or  very 
slightly  active  substance  to  a  more  active  one  to  produce  the 
increased  activity.  These  phenomena  may  be  very  easily  ex- 
plained on  an  assumption  of  the  following  nature :  Suppose  that 
the  active  deposit  is  not  a  simple  substance  but  a  complex 
one  consisting  of  a  mixture  of  at  least  two  substances  which 
have  been  named  by  Rutherford  thorium  A  and  thorium  B,  and 
suppose  that  thorium  A  arises  directly  from  the  emanation  and 
is  first  deposited  on  the  wire  and  then  changes  into  thorium  B, 
and  finally  thorium  B  then  changes  into  something  else.  For 
a  short  exposure  the  deposit  will  consist  almost  entirely  of 
thorium  A,  as  very  little  of  it  has  had  time  to  change  into 
thorium  B,  If  we  also  suppose  that  thorium  A  either  gives 


TRANSFORMATION    PRODUCTS   OF    THORIUM  211 

out  no  rays  at  all  or  rays  which  produce  a  very  small  amount 
of  ionization  compared  with  those  from  thorium  B,  then  the 
activity  at  first  will  be  very  small,  due  almost  entirely  to  the 
very  small  portion  of  thorium  B  present.  If  thorium  A 
changes  into  thorium  B,  which  in  turn  decays,  then  the  activity 
will  increase  until  a  maximum  is  reached,  when  the  change  of 
A  into  B  just  balances  the  decay  of  B.  Then  beyond  this  stage 
as  more  atoms  of  B  will  change  per  second  than  are  produced 
from  A  the  total  activity  will  gradually  decrease.  In  the  case 
of  the  long  exposure  this  maximum  has  been  reached  before 
removal  from  the  emanation,  and  consequently  no  initial  rise  is 
observed. 

It  was  thought  for  some  time  after  discovery  that  thorium 
A  emitted  no  rays  at  all  and  it  was  termed  a  "  rayless  "  product, 
but  it  has  recently  been  shown  that  it  emits  a  slow-moving  type 
of  ft  rays  whose  ionizing  power  is  very  weak.  These  two 
substances  have  been  separated  by  special  means  and  have  been 
shown  to  be  two  distinct  substances  which  have  distinct  periods 
of  change,  thorium  A  being  half  transformed  in  eleven  hours 
while  thorium  B  requires  only  one  hour. 

Still  more  recently  it  has  been  shown  that  thorium  B  is 
itself  not  a  simple  substance,  but  consists  of  at  least  two  sub- 
stances which  have  been  called  thorium  B  and  thorium  C,  the 
former  being  the  parent  of  the  latter.  Some  very  recent 
experiments  of  Hahn  indicate  that  this  list  may  be  still  further 
extended  to  include  another  product  called  thorium  D.  Thor- 
ium B  emits  only  a  rays,  while  thorium  C  emits  not  only  a  rays 
but  very  probably  ft  and  y  rays  also. 

140.  Decay  and  Rise  of  the  Excited  Activity  from  Radium. 
.  — An  examination  of  the  active  deposit  from  radium  emana- 
tion shows  an  even  more  complicated  state  of  affairs  than  exists 
among  the  thorium  products.  The  decay  curves  measured  by 
the  different  types  of  rays  show  distinct  differences  and  even 
the  form  of  the  curves  indicate  peculiarities  not  present  in  the 
curves  for  thorium. 

The  following  method  of  obtaining  the  active  deposit  from 
radium  for  a  series  of  experiments  will  be  found  most  con- 


212 


EXCITED  ACTIVITY 


venient.  Dissolve  a  quantity  of  radium  bromide  in  water  in  a 
bottle  of  about  three  times  the  capacity  of  the  solution.  Fit 
the  neck  of  the  bottle  with  an  ebonite  stopper. 
Through  a  hole  in  this  stopper  pass  a  wire  A 
of  somewhat  smaller  diameter  than  the  hole  as 
shown  in  Fig.  69,  and  on  this  wire  fit  a  collar 
which  will  allow  the  wire  to  pass  through  the 
stopper  to  a  definite  distance  above  the  solution. 
When  in  place  the  openings  in  and  around  the 
stopper  may  be  sealed  with  wax.  Connect  this 
central  wire  A  to  the  negative  pole  of  a  battery 
of  two  or  three  hundred  volts,  while  the  other 
pole  of  the  battery  is  connected  through  a  large 
resistance  to  earth.  Connect  the  solution  in  the 
bottle  to  earth  by  the  wire  B  which  dips  into 
the  solution.  The  emanation  collects  in  the 
space  in  the  bottle  above  the  solution  and  the 
active  deposit  collects  on  the  charged  wire 
suspended  above  the  dissolved  radium. 

Expose  this  wire  for  about  twenty-four  hours  to  the  emana- 
tion. Remove  it  and  make  it  the  central  electrode  A  in  a  metal 
cylinder  of  about  4  cm.  diameter,  as  shown  in  Fig.  70.  Make 
connections  as  shown  in  the  diagram.  Around  the  electrode  A 


EARTH 


FIG.  69. 


EARTH 


EARTH 

FIG.  70. 


and  insulated  from  it  and  from  the  cylinder  is  a  metal  guard- 
ring  rr  connected  to  earth  to  prevent  any  leakage  from  the 
battery  to  the  wire.  Using  the  steady  deflection  method  meas- 


EXCITED   ACTIVITY  FROM    RADIUM  213 

ure  the  activity  of  A  at  intervals  of  about  two  minutes  for  a 
period  of  about  one  and  a  half  hours  and  plot  the  usual 
decay  curve.  In  the  meantime,  while  these  measurements  are 
being  made,  expose  a  similar  wire  for  a  period  of  about  an 
hour  and  then  determine  its  decay  curve  as  soon  as  the  first 
determination  is  finished.  Make  a  third  exposure  of  ten 
minutes  and  determine  the  corresponding  curve.  Plot  these 
curves  on  the  same  scale  and  compare  them.  The  ionization 
current  in  these  cases  is  due  almost  entirely  to  a  rays  which 
might  be  tested  if  desired  by  the  usual  absorption  method. 
Note  the  rapid  initial  drop  in  the  curves  and  then  the  slower 
rate  of  decay  for  a  time,  and  then  afterwards  the  rate  begins 
to  increase  again.  Observe  that  the  shorter  the  exposure  the 
slower  is  the  rate  of  decay  after  the  initial  rapid  drop. 

Now  repeat  these  experiments  for  exactly  the  same  times 
of  exposure,  but  instead  of  measuring  the  activity  by  means  of 
the  a  rays,  using  an  electrometer,  test  the  activity  by  means  of 
the  ft  rays  emitted  by  the  active  deposit.  To  do  this  use  an 
electroscope  of  the  type  shown  in  Fig.  14,  and  cut  a  hole 
in  the  bottom  of  this  electroscope  and  cover  it  with  a  sheet 
of  aluminium  foil  thick  enough  to  absorb  all  the  a  rays,  and 
then  place  the  active  wire  just  underneath  the  foil  which  will 
allow  the  ft  and  y  rays  to  pass  through  and  ionize  the  air 
inside  the  electroscope.  The  ft  rays  will  however  produce  by 
far  the  greater  part  of  the  ionization.  The  decay  curves 
measured  by  the  ft  rays  will  be  found  quite  different  from 
those  measured  by  the  a  rays.  The  ft  ray  activity  of  the 
deposit  obtained  by  long  exposure  will  begin  to  decay  at  once, 
but  comparatively  slowly,  while  the  activity  of  the  deposits  by 
short  exposure  will  at  first  increase  to  a  maximum  and  then 
decrease.  The  shorter  the  exposure  the  greater  will  be  the 
increase,  but  after  the  maximum  is  reached  the  rate  of  decrease 
will  be  practically  the  same  for  all  exposures,  even  for  the 
deposit  by  long  exposure. 

These  experiments  should  be  repeated  once  more,  measuring 
the  activity  of  the  deposit  by  the  y  rays  alone.  To  do  this 
set  the  electroscope  on  a  lead  plate  about  6  mm.  thick,  which 


214  EXCITED  ACTIVITY 

will  absorb  both  the  a  and  /?  rays,  and  the  ionization  in  the 
electroscope  will  be  due  to  the  y  rays  alone.  Obtain  curves 
for  exactly  the  same  times  of  exposure  using  the  y  rays. 
These  curves  will  be  found  to  be  identical  with  those  obtained 
from  the  (3  rays.  This  shows  that  whatever  the  changes  tak- 
ing place  the  /?  and  y  rays  always  occur  together. 

141.  Explanation  of  the  Decay  Curves  of  the  Active  Deposit 
from  Radium. — The  peculiar  shape  of  the  decay  curves  as 
determined  by  the  a  rays  along  with  the  totally  different  shape 
of  those  determined  by  the  /3  and  y  rays  indicate  that  the 
active  deposit  from  radium  must  be  very  complex.  The  a  ray 
curves  all  show  a  very  rapid  initial  decay  for  about  the  first 
ten  minutes,  followed  by  a  very  much  slower  rate  for  about 
thirty  or  thirty-five  minutes,  and  this  in  turn  is  followed  by  a 
somewhat  more  rapid  rate  of  decay.  The  (3  and  y  ray  curves 
show,  except  in  the  case  of  long  exposure,  an  initial  rise  in 
activity  for  about  fifteen  or  twenty  minutes,  followed  by  a 
decay.  By  a  process  of  analysis  partly  theoretical  and  partly 
experimental  and  of  a  somewhat  lengthy  nature,  the  details 
of  which  are  however  beyond  the  scope  of  this  course,  it  has 
been  shown  that  the  active  deposit  from  radium  emanation 
consists  in  the  first  instance  of  three  distinct  substances  which 
have  been  named  radium  A,  radium  B  and  radium  C.  Radium 
A  emits  only  a  rays  and  in  decaying  changes  into  radium  B, 
requiring  only  three  minutes  to  be  half  transformed.  Radium 
B  emits  (3  and  y  rays  and  has  a  period  of  twenty-six  minutes, 
while  radium  C  gives  out  all  three  types  of  radiations  and  is 
half  transformed  in  nineteen  minutes.  The  fact  that  radium 
A  emits  no  ft  or  y  rays  accounts  for  the  initial  rise  in  the 
/?  or  y  ray  curves  for  short  exposure.  For  a  short  exposure 
most  of  the  deposit  consists  of  radium  A,  and  therefore  the 
13  or  y  ray  activity  is  very  small,  due  only  to  the  small 
amount  of  radium  B  or  C  present.  Radium  A  changes 
rapidly  into  radium  B,  which  in  turn  changes  into  radium  C, 
both  of  which  give  out  ft  and  y  rays,  and  therefore  the 
/3  and  y  ray  activity  increases,  until  finally  the  rate  of  decay 
of  radium  C  is  greater  than  the  rate  of  supply,  and  therefore 
there  is  a  total  gradual  decrease. 


ACTIVE  DEPOSIT  OF   SLOW    CHANGE  215 

In  the  case  of  the  a  ray  curves  the  a  radiation  from  radium 
A  rapidly  decays  as  radium  A  changes  into  radium  B  which 
gives  out  no  a  rays,  but  it  in  turn  very  soon  changes  into 
radium  C,  which  again  increases  the  supply  of  a  rays  and 
prevents  the  decay  of  the  a  radiation  from  being  so  rapid. 
But  finally  all  the  radium  A  and  B  are  changed  into  radium  C, 
and  as  it  changes  the  a  ray  activity  again  diminishes  more 
rapidly. 

142.  Active  Deposit  of  Slow  Change. — After  the  greater 
portion  of  the  excited  activity  of  the  deposit  from  radium  has 
disappeared,  which,  as  we  have  seen,  takes  place  in  a  few 
minutes,  there  remains  a  residual  activity  which  decays  ex- 
tremely slowly.  This  residual  activity  is  only  a  very  small 
fraction  of  the  original  activity  of  the  deposit  immediately 
after  removal  from  the  emanation.  It  varies  somewhat  with 
the  conditions  but  is  of  the  order  of  about  1/300,000  part  of 
the  original  activity,  being  under  some  circumstances  much  less 
than  this  and  under  others  greater. 

If  a  platinum  plate  be  exposed  to  the  radium  emanation  for 
a  period  of  about  a  week  and  then  removed  and  its  activity 
tested  the  rapid  decay  of  the  activity  already  studied  will  be 
observed.  After  an  interval  of  two  or  three  days  this  rapidly 
decaying  activity  will  have  all  disappeared,  but  a  small  residual 
will  remain.  If  this  residual  activity,  as  measured  by  the 
a  rays,  be  measured  it  will  be  found  to  increase  very  slowly, 
and  continue  to  increase  for  several  months,  but  finally,  after 
a  long  period,  will  approach  a  maximum.  If,  however,  the 
activity  be  measured  by  the  (3  rays  it  reaches  a  maximum  in  a 
much  shorter  interval  of  only  about  a  month.  These  results, 
along  with  other  determinations  extending  over  lengthy 
periods,  indicate  that  this  residual  active  matter  is  a  complex 
substance,  consisting  of  slowly  changing  products.  The  results 
of  experiments  up  to  date  show  that  this  residual  deposit  con- 
sist of  four  distinct  products,  which  by  analogy  have  been 
named  radium  D,  radium  E,  radium  F  and  radium  G. 

The  periods  of  transformation  for  these  four  products  are 
very  slow  compared  with  those  for  radium  A,  B  and  C.  Ra- 


2l6  EXCITED  ACTIVITY 

dium  D  and  radium  E  do  not  emit  any  rays,  and  are  therefore 
rayless  products  and  are  half  transformed  in  forty  years  and 
six  days  respectively.  Radium  F  emits  (3  -and  y  rays  and  has 
a  period  of  four  and  a  half  days,  while  radium  G  emits  only  a 
rays  and  its  period  is  140  days.  These  four  products  are 
called  collectively  the  active  deposit  of  slow  change,  while 
radium  A,  B  and  C  form  a  group  called  the  active  deposit  of 
rapid  change. 

143.  Active  Deposit  from  Actinium. — A  negatively  charged 
wire  exposed  to  the  emanation  of  actinium  receives  an  active 
deposit  possessing  properties  very  similar  to  the  active  deposit 
from  thorium.     This  active   deposit   from  actinium  may  be 
studied  experimentally  in  a  manner  similar  to  that  described 
in  the  case  of  thorium,  and  it  will  be  found  that  it  is  also  com- 
plex, consisting  of  three  distinct  products  which  have  been 
called"  actinium  A,  actinium  B  and  actinium  C.     The  periods  of 
transformation  are  all  comparatively  short,  being  36  minutes, 
2.15  minutes  and  5.1  minutes,  respectively.     Actinium  A  emits 
/?  rays,  actinium  B  only  a  rays,  while  actinium  C  emits  ft  and 
y  rays. 

144.  Some  General  Properties  of  the  Active  Deposits. — If 
a  wire  which  is  to  be  made  radio-active  is  carefully  weighed 
before  and  after  exposure  to  the  emanation  no  difference  in 
weight  will  be  detected,  no  matter  how  long  the  exposure  may 
be,  or  if  the  active  wire  be  examined  under  the  microscope  no 
trace  of  foreign  matter  will  be  observed.     These  facts  alone 
might  lead  to  the  conclusion  that  what  has  been  called  a  de- 
posit is  really  not  of  a  material  nature.     -Other  facts,  however, 
show  that  it  is  really  a  material  deposit,  but  is  extremely  minute 
in  quantity  and  it  would  in  all  probability  escape  detection  if 
it  were  not  for  its  power  of  emitting  radiations.     If  a  wire 
carrying  an  active  deposit  from  radium  be  drawn  across  a  zinc 
sulphide  screen  a  brightly  luminous  trace  is  left  behind,  which 
continues  for  some  time,  indicating  that  some  of  the  active 
matter  is  rubbed  off  and  left  on  the  phosphorescent  screen. 
The  deposit  may  also  be  partially  rubbed  off  the  wire  by  rub- 
bing with  a  cloth,  and  almost  completely  removed  by  rubbing 


TRANSFORMATION    PRODUCTS  217 

with  sand  paper,  and  after  rubbing  the  deposit  will  be  found 
on  the  sand  paper. 

A  large  portion  of  the  deposit  may  also  be  dissolved  off  the 
wire  by  dipping  the  wire  in  hydrochloric  or  sulphuric  acid.  If 
the  acid  be  then  evaporated  the  active  deposit  is  left  behind  on 
the  vessel  and  has  not  been  altered  in  its  properties.  The 
active  deposit  decays  at  the  same  rate  while  in  solution  in  acid 
as  it  does  in  the  undissolved  state. 

If  a  wire  containing  the  active  deposit  be  heated  to  a  white 
heat  the  activity  disappears  from  the  wire.  The  activity  is  not 
destroyed,  for  if  the  wire  be  surrounded  by  a  cylinder  while 
being  heated  the  activity  will  be  found  on  the  cylinder  after 
heating.  This  shows  that  at  this  high  temperature  the  active 
deposit  becomes  volatile  and  is  transferred  from  the  wire  to 
the  cylinder. 

145.  Some  other  Transformation  Products. — Recent  inves- 
tigations, some  of  which  have  extended  over  a  considerable 
time,  have  revealed  still  other  intermediate  transformation 
products  belonging  to  these  already  large  families  of  radio- 
active substances.  An  intensely  active  body  which  has  been 
called  radiothorium  has  been  found  in  the  thorium  family.  It 
is  intermediate  between  thorium  and  thorium  X,  and  is  the 
immediate  parent  of  thorium  X.  It  emits  a  rays  and  has  a 
period  of  transformation  of  800  days. 

In  addition  to  this  there  is  another  body  existing  between 
thorium  and  radiothorium,  which  was  at  first  thought  to  be  a 
single  substance  and  was  called  mesothorium,  but  quite  recently 
this  has  been  still  further  analyzed  by  Hahn  and  found  to  be 
complex,  consisting  of  two  substances  for  which  the  names 
mesothorium  I  and  mesothorium  2  have  been  suggested. 
Mesothorium  i  arises  directly  from  thorium  and  is  a  rayless 
body  having  a  period  of  5.5  years  and  gives  rise  to  meso- 
thorium 2  which  emits  ft  and  y  rays.  Mesothorium  2  is  half 
transformed  in  about  6.2  hours,  and  in  so  doing  changes 
directly  into  radiothorium. 

In  the  actinium  series  there  exists  between  actinium  and 
actinium  X  a  substance  of  similar  properties  to  radiothorium 


2l8  EXCITED  ACTIVITY 

which  from  analogy  has  been  called  radioactinium.  It  emits 
a  rays  and  is  the  immediate  parent  of  actinium  X. 

One  of  the  most  recent  discoveries  in  this  connection  is  that 
of  a  product  which  finds  a  place  in  the  series  between  uranium 
and  uranium  X.  It  has  been  proposed  to  call  it  radio-uranium, 
and  it  is  considered  to  be  the  immediate  parent  of  uranium  X. 
Its  properties  have  not  as  yet  been  completely  investigated. 

From  a  variety  of  indirect  evidence  it  has  for  a  long  time 
been  thought  that  there  was  a  close  relation  between  uranium 
and  the  radium  family.  Many  attempts  have  been  made  to 
prove  this  experimentally,  but  with  somewhat  indifferent  suc- 
cess. Recently  however  an  active  body  which  has  been  called 
ionium  has  been  located  in  uranium  minerals  and  has  been 
pretty  well  proved  to  be  the  connecting  link.  It  is  thought  that 
this  ionium  arises  directly  from  uranium  X,  and  is  in  turn 
itself  the  immediate  parent  of  radium.  The  uranium  and 
radium  families  then  form  one  complete  series  with  ionium 
as  the  connecting  link. 

146.  Theory  of  Radio-active  Changes. — From  a  study  of 
the  various  radio-active  substances  individually  and  in  their 
relation  to  one  another  we  have  seen  that  there  are  a  series 
of  continuous  changes  from  one  substance  to  another  taking 
place  which  so  far  have  never  been  observed  in  any  other  class 
of  materials.  Each  of  these  substances  is  entirely  distinct  from 
the  others  and  has  distinct  physical  and  chemical  properties. 
They  are  not  permanent  substances  however,  but  gradually 
decay,  and  each  one  has  a  distinct  and  definite  period  of  decay 
which  is  its  most  distinguishing  property  and  by  which  it  is 
differentiated  from  all  the  others.  These  changes  from  one 
substance  to  another  take  place  without  the  aid  of  any  outside 
agency  and  in  fact  are  practically  unaffected  by  any  external 
influence.  How  do  these  remarkable  changes  come  about? 
Rutherford  and  Soddy  were  the  first  to  offer  a  satisfactory 
theory  to  explain  these  phenomena.  The  disintegration  theory 
or  theory  of  successive  changes  which  they  put  forth  in  the 
year  1902  and  which  has  since  been  developed  is  now  the  gen- 
erally accepted  one. 


DISINTEGRATION    THEORY  219 

Radio-active  properties  have  been  shown  to  be  properties 
of  the  atom  and  not  of  the  molecule,  for  the  activity  of  radium, 
for  instance,  depends  upon  the  amount  of  the  element  present 
and  not  upon  its  chemical  combination  with  other  elements. 
According  to  the  general  theory  of  J.  J.  Thomson  an  atom  of 
any  substance  may  be  considered  a  complex  structure  con- 
sisting of  positively  and  negatively  charged  particles  in  rapid 
rotation  within  its  own  system  and  held  together  by  their 
mutual  forces  in  equilibrium.  For  such  a  system  to  be  perma- 
nent it  must  be  in  stable  equilibrium.  According  to  the  disin- 
tegration theory  this  complex  structure,  constituting  the  atom  of 
radium  (which  we  shall  take  as  the  typical  example),  becomes 
by  some  means  unstable  and  one  of  the  positively  charged  a 
particles  is  suddenly  expelled  with  great  velocity.  This  a  par- 
ticle constitutes  the  radio-active  radiation  from  radium.  The 
structure  of  the  atom  which  remains  is  now  different  and 
constitutes  the  atom  of  a  new  substance,  namely,  the  emana- 
tion. These  atoms  of  the  emanation  are  unstable  and  gradu- 
ally change  by  the  expulsion  of  another  a  particle  from  each 
atom.  The  remaining  atom  now  in  turn  constitutes  the  atom 
of  a  new  substance,  namely,  radium  A.  These  atoms  in  their 
turn  break  up  as  before,  changing  into  radium  B  and  the 
process  is  continued  throughout  the  successive  changes.  The 
processes  are  not  identical  in  all  instances,  for  in  some  cases 
an  a  particle  alone  is  expelled  but  in  others  negatively  charged 
/?  particles  are  expelled  accompanied  by  y  rays,  while  in  other 
cases  the  change  consists  in  giving  out  all  three  types,  a,  ft 
and  y  rays.  The  production  of  one  radio-active  product  from 
another  consists,  according  to  this  theory,  in  the  disintegration 
of  the  atom  by  the  expulsion  of  a  positively  or  negatively 
charged  particle  or  both  owing  to  the  system  of  which  the  atom 
is  composed  becoming  for  some  reason  unstable.  The  radio- 
active property  of  the  substances  is  the  result  of  this  disinte- 
gration by  expulsion  of  charged  particles.  The  radio-activity 
is  an  accompaniment  of  the  disintegration  of  the  atoms. 

Why  do  these  atoms  suddenly  become  unstable  and  break 
up  without  any  apparent  cause?  Several  explanations  have 


220  EXCITED  ACTIVITY 

been  offered  to  account  for  this,  but  the  most  probable  one 
seems  to  be  that  if  this  system  of  charged  particles,  of  which 
the  atom  in  all  probability  consists,  is  in  rapid  rotation  it  must 
be  radiating  energy,  and  when  sufficient  energy  has  been  radi- 
ated the  mutual  forces  of  the  system  no  longer  balance  and 
one  or  more  of  the  particles  escape  and  cause  disintegration. 

These  atoms  of  the  radio-active  substances  have  an  indepen- 
dent existence  and  distinct  physical  and  chemical  properties, 
but  they  differ  from  the  atoms  of  ordinary  non-radio-active 
elements  in  the  fact  that  they  are  not  permanent.  These  pro- 
ducts are  however  just  as  truly  elements  while  they  last  as  are 
the  non-radio-active  elements.  To  distinguish  these  unstable 
atoms  from  the  ordinary  atoms  the  term  metabolon  has  been 
suggested  as  a  convenient  name. 

We  have  observed  in  a  few  instances  that  the  transformation 
products  do  not  emit  any  rays  at  all,  and  the  change  from  them 
into  the  succeeding  substance  apparently  takes  place  without 
the  expulsion  of  any  particles.  These  "  rayless  "  changes  may 
be  explained  in  either  of  two  ways.  The  new  product  may  be 
formed  simply  by  the  rearrangement  of  the  system  of  charged 
particles  among  themselves  within  the  atom  without  the  ex- 
pulsion of  any  of  them.  The  new  atom  may  consist  of  the 
same  set  of  particles  arranged  under  a  new  system.  This 
rearrangement  may  not  take  place  with  sufficient  violence  to 
expel  any  of  the  members  of  the  system. 

Another  hypothesis  which  would  explain  the  rayless  change 
is  that  it  may  be  caused  by  the  expulsion  of  one  or  more 
charged  particles,  but  with  a  velocity  too  slow  to  ionize  the  gas. 
Experiments  show  that  when  the  velocity  of  the  a  particle 
falls  below  io9  cm.  per  second  it  ceases  to  ionize  the  gas, 
and  consequently  an  a.  particle  expelled  with  a  velocity  below 
this  minimum  would  escape  detection,  since  it  produces  no  ions. 
The  so-called  rayless  change  thus  might  be  caused  by  the  ex- 
pulsion of  an  a  particle  with  a  velocity  below  the  minimum 
necessary  to  produce  ions.  This  hypothesis  receives  some  sup- 
port from  the  fact  that  in  a  few  instances  certain  products 
were  considered  for  a  time  to  be  rayless,  but  further  investiga- 


RADIO-ACTIVE   ELEMENTS  221 

tion  revealed  the  existence  of  slow-moving  charged  particles 
arising  from  them,  which,  being  very  weak  ionizers,  had 
escaped  detection. 

The  latter  hypothesis  suggests  the  possibility  that  all  matter 
might  be  undergoing  a  very  slow  change  in  a  similar  manner, 
and  that  the  only  reason  this  change  has  been  observed  in  the 
so-called  radio-active  bodies  and  not  in  the  non-radio-active 
bodies  is  that  in  the  case  of  the  former  the  charged  particles  are 
expelled  with  sufficient  violence  to  ionize  the  gas,  while  in  the 
latter  case  they  may  be  expelled  but  not  with  sufficient  violence 
to  produce  ions.  To  decide  this  question  experimentally  ap- 
pears to  be  practically  impossible  unless  some  means,  other  than 
at  present  known,  of  detecting  such  slow-moving  particles  be 
discovered. 

147.  Summary  of  Radio-active  Elements. — Since  the  dis- 
covery of  radio-activity  the  list  of  radio-elements  has  grown 
rapidly.  Owing  to  the  persistent  energy  of  experimenters  in 
this  line  the  list  is  continually  receiving  additions,  and  in  all 
probability  there  still  remain  others  to  be  discovered.  In  fact, 
during  the  writing  of  this  book  two  new  radio-elements  have 
been  added  to  the  list.  The  following  table  contains  a  sum- 
mary of  all  the  radio-active  elements  at  present  known  with 
some  of  their  distinguishing  characteristics.  On  account  of 
the  incomplete  state  of  the  subject  further  experimental  inves- 
tigation and  still  more  accurate  determinations  will  no  doubt 
cause  additions  and  alterations  in  connection  with  this  table. 

The  first  column  gives  the  elements  in  the  order  in  which 
they  are  transformed  from  one  to  the  other;  the  second  column 
shows  the  period  required  for  the  transformation  to  be  half 
completed,  while  the  third  indicates  the  nature  of  the  rays 
emitted  by  each  product. 


222 


EXCITED  ACTIVITY 
TABLE   OF  RADIO-ELEMENTS 


Radio-Active 
Products. 

Transfor- 
mation 
Period. 

Nature 
of  Rays 
Emitted. 

Radio-Active 
Products. 

Transfor- 
mation 
Period. 

Nature 
of  Rays 
Emitted. 

Uranium 

5Xio9years 

a 

Thorium 

I010  years 

a 

4 

4 

Radiouranium 

? 

? 

Mesothorium  i 

5.5  years 

Rayless 

4 

4 

Uranium  X 

22  days 

,Sand  7 

Mesothorium  2 

6.2  hours 

P  and  7 

* 

4 

Ionium 

? 

ot 

Radiothorium 

800  days 

oc 

4 

4 

Radium 

2000  years 

ft 

Thorium  X 

3-7  days 

a 

4 

4 

Emanation 

3.75  days 

a. 

Emanation 

54  seconds 

a 

4 

4 

Radium  A 

3  minutes 

a 

Thorium  A 

II  hours 

Slow/3  rays 

4 

4 

Radium  B 

26  minutes 

p  and  7 

Thorium  B 

I  hour 

a 

4 

4 

Radium  C 

19  minutes 

a,  P,  and  / 

Thorium  C 

? 

a,  p,  and  7 

4 

4 

Radium  D 

40  years 

Rayless 

Thorium  D 

? 

? 

(Radio-lead) 

4 

Radium  E 

6  days 

Rayless 

Actinium 

? 

Rayless 

Radium  F 

4.5  days 

P  and  7 

Radioactinium 

19.5  days 

a 

Radium  G 
(Polonium) 
I 

140  days 

a 

Actinium  X 

4 

lo  days 

a 

v 

Emanation 

3.7  seconds 

a 

4 

Actinium  A 

36  minutes 

P 

Actinium  B 

2.15  minutes 

a. 

4 

Actinium  C- 

5.1  minutes 

P  and  7 

CHAPTER  XV. 

A  SPECIAL  METHOD  OF  ANALYSIS  BY  ABSORPTION 
CURVES. 

148.  In  the  present  chapter  a  brief  account  will  be  given  of 
a   special  experimental  method  of  radio-active  measurement 
which  has  been  used  to  great  advantage  and  which  has  led  in 
some  instances  to  important  discoveries. 

149.  Homogeneous  Source  of  a  Rays. — In  several  important 
investigations  on  a  rays,  especially  those  involving  accurate 
quantitative  determinations  of  their  constants,  it  is  very  neces- 
sary to  have  a  homogeneous  source  of  a  rays.     This  cannot 
be  obtained   from  the  ordinary  radium  compounds  in  equi- 
librium, for  these  compounds  are  complex,  the  different  con- 
stituents emitting  rays  of  different  velocities.     A  very  con- 
venient source  is   furnished  by  the  active  deposit  from  the 
radium  emanation.     If  a  fine  copper  wire  be  exposed  to  the 
radium  emanation  for  about  two  hours  an  active  deposit,  con- 
sisting of  radium  A,  B  and  C,  is  the  result,  of  which  radium 
B  emits  no  a  rays,  so  that  all  the  a  rays  come  from  radium 
A  and  C.     But  the  activity  of  radium  A  very  rapidly  decays, 
reaching  half  value  in  three  minutes,  so  that  at  the  end  of 
about  fifteen  minutes  after  removal  from  the  emanation  ra- 
dium A  has  practically  disappeared  and  therefore  radium  C 
is  the  only  source  of  a  rays,  which  thus  furnishes  a  homo- 
geneous source. 

Radium  C  deposited  in  this  manner  has  other  advantages  as 
well.  It  may  be  deposited  on  a  very  fine  wire,  and  therefore 
a  source  of  very  small  dimensions  may  be  used.  In  addition, 
the  coating  of  deposit  on  the  wire  is  so  extremely  thin  that 
there  is  no  absorption  of  the  rays  due  to  passage  through  the 
active  material. 

There  is  one  slight  disadvantage  in  regard  to  this  source 
owing  to  the  fact  that  its  activity  decays  with  time.  This  how- 
ever is  not  serious,  as  the  rate  of  decay  has  been  so  carefully 

223 


224  ANALYSIS   BY   ABSORPTION    CURVES 

determined  and  this  determination  may  be  repeated  in  each 
case  simultaneously  with  the  other  measurements  if  necessary, 
that  the  rate  of  decay  is  known  and  thus  the  readings  can 
always  be  corrected  for  a  constant  source. 

This  source  of  a  rays  has  been  utilized  in  a  number  of  im- 
portant determinations,  such  as  the  magnetic  and  electrostatic 
deflection  of  the  a  rays  in  the  determination  of  the  velocity 
and  the  value  of  e/m  (see  §no),  and  the  determination  of 
the  range  in  air  over  which  these  rays  produce  ionization,  or 
photographic  or  phosphorescent  action.  The  change  in  veloc- 
ity and  the  absorption  of  the  a  rays  in  passing  through  matter 
has  also  been  determined  by  using  such  a  source. 

150.  Theory  of  Analysis  by  Absorption  Curves. — We  have 
seen  that  the  a  rays  are  absorbed  in  passing  through  gases  and 
that  it  requires  only  a  comparatively  few  centimeters  of  air  to 
completely  absorb  them,  that  is,  they  have  a  definite  range 
in  air.  The  a  particle  in  passing  through  the  gas  ionizes  it, 
and  as  ionization  requires  energy  the  energy  is  obtained  at 
the  expense  of  the  kinetic  energy  of  the  moving  particle,  and 
finally  its  velocity  is  reduced  below  that  required  to  produce 
ions  and  complete  absorption  is  the  result,  as  it  is  not  observ- 
able after  it  has  lost  its  power  to  produce  ions.  The  range  of 
complete  absorption  will  be  different  for  the  a  rays  from 
different  substances,  since  they  are  projected  with  different 
velocities. 

Suppose  that  a  thick  layer  of  a  single  radio-active  element 
R  (Fig.  71)  be  placed  in  the  bottom 

A of  a  deep  recess  in  a  lead  block  so  as 

~P -----  j-o  obtain  a  definite  cone  of  a  rays. 

Let  A,  a  metal  plate,  and  B,  a  sheet 
of  wire  gauze  parallel  to  A  at  a 
fixed  distance  from  it  of  2  or  3  mm., 
form  an  ionization  chamber.  The  a 
rays  from  R  come  from  different 
depths  in  the  material,  and  therefore 
those  arising  from  the  lower  layers 
will  be  absorbed  more  before  they 


ANALYSIS    BY   ABSORPTION    CURVES 


225 


emerge  into  the  air  than  those  from  the  layers  nearer  the 
surface.  Consequently  a  rays  of  velocities  extending  over 
a  considerable  range  will  emerge  at  the  surface  and  ionize 
the  air.  If  the  chamber  AB  be  placed  above  the  source  at 
a  distance  beyond  the  range  of  the  swiftest  particles  no 
ionization  will  be  produced  in  AB  since  no  rays  will  reach 
it  with  sufficient  velocity  to  ionize.  The  area  of  both  A 
and  B  should  be  great  enough  to  cover  more  than  the  whole 
cross-section  of  the  cone  of  rays.  If  AB  is  gradually  lowered 
the  rays  of  greatest  range  will  first  enter  it  and  produce  a 
small  amount  of  ionization,  and  on  lowering  it  still  farther  the 
rays  of  next  greatest  range  will  reach  it  and  increase  the 
ionization.  By  continuing  to  lower  AB  the  ionization  will 
gradually  increase,  because  more  and  more  rays  enter  it  the 
nearer  it  is  to  the  source.  Consequently,  if  a  curve  be  plotted 
showing  the  relation  between  ionization  and  distance  from  the 
source  it  would  take  the  general  form  of  the  straight  line  MN, 
Fig.  72,  where  the  ordinates  represent  distance  from  the  source 
and  the  abscissae  the  ionization.  The  ionization  increases  as 
the  source  is  approached. 

If  however  a  thin  layer  of  ma- 
terial be  used  at  R  instead  of  the  M 
thick  one  the  rays  will  be  much 
more  nearly  homogeneous.  The 
gradual  increase  will  take  place 
over  a  much  shorter  range  and 
within  a  certain  distance  of  the 
source  all  the  rays  will  enter  AB 
and  the  curve  will  take  the  form 

MPR.    At  the  distance  correspond-  R  # 

ing  to  P  all  the  rays  enter  AB  and  FlG>  72> 

therefore   the   ionization   does   not 
increase  any  more  by  further  approach  towards  the  source. 

If  a  thin  layer  of  a  complex  radio-active  body  consisting  of 
two  or  more  active  elements,  from  each  of  which  a  group  of 
rays  arises  with  initial  velocity  different  from  the  others,  be 
used  as  source  the  curve  will  be  a  little  more  complicated  and 
will  take  the  general  form  shown  in  Fig.  73,  supposing  the 


226 


ANALYSIS    BY   ABSORPTION    CURVES 


source  consists  of  a  mixture  of  three  simple  substances.  The 
part  RS  corresponds  to  the  rays  from  one  source  which  are  of 
greatest  range,  and  if  these  were  the  only  ones  present  the 
curve  would  continue  along  the  dotted  line  SA.  At  the  distance 

corresponding  to  51  the 
group  of  rays  from  the 
second  source  of  next 
greatest  range  make 
their  appearance  in  the 
ionization  chamber  and 
constitute  the  part  ST 
which,  if  there  were  no 
more,  would  continue 
along  TB.  Similarly  the 
third  source  corresponds 
to  the  part  TV.  This 
method  of  investigation  furnishes  a  powerful  method  of 
analysis  in  certain  lines,  and  by  its  application  the  presence  -of 
some  of  the  radio-elements  have  been  first  discovered. 

151.  Experimental  Arrangement  for  Analysis. — The  appa- 


3 

FIG.  73. 


fe* 


H 


m. 


lAKTtt 


CARTS' 


FIG.  74. 

ratus    shown    in    Fig.    74    has    been    found    to    be    suitable 


ANALYSIS    BY   ABSORPTION    CURVES  227 

for  this  type  of  investigation.  PM  is  a  metal  box  about  25 
cm.  square.  A  is  the  upper  plate  of  the  ionization  chamber 
consisting  of  a  central  plate  about  12  cm.  square  surrounded 
by  a  guard-ring.  The  central  plate  is  insulated  and  supported 
by  the  ebonite  arches  c  and  d.  B  is  a  sheet  of  wire  gauze 
of  fine  wire  but  wide  meshes  of  about  2  or  3  mm.  supported 
and  insulated  by  ebonite  blocks,  as  shown,  at  a  distance  of  3 
mm.  from  A.  This  whole  system  is  supported  from  the  cover 
of  the  box. 

The  receptacle  R  for  holding  the  radio-active  body  consists 
of  a  brass  block  about  3  cm.  square  made  in  two  sections.  The 
lower  half  has  a  shallow  recess  cut  in  the  center  of  the  upper 
face,  while  the  upper  half  has  a  corresponding  central  hole 
of  less  diameter  than  the  recess  in  the  lower  section.  The 
two  parts  are  made  to  fit  together  in  a  fixed  relative  position. 
They  may  be  separated  and  the  radio-active  body  placed  in 
the  recess  of  the  lower  section  and  then  the  upper  one  placed 
over  it  so  that  the  radiations  can  emerge  only  through  the  hole 
in  the  upper  block.  This  is  supported  on  an  upright  H  which 
passes  through  an  opening  in  the  bottom  of  the  box,  and  which 
may  be  moved  vertically  by  a  screw  and  the  distance  of  R  from 
A  measured  by  a  scale  along  the  side.  The  source  of  rays 
can  thus  be  placed  at  any  distance  from  A  and  the  ionization 
in  AB  measured  in  the  usual  manner. 

Place  about  5  milligrams  of  radium  bromide  in  R  and  meas- 
ure the  saturation  current  between  A  and  B  for  different  dis- 
tances of  R  from  A  over  a  range  of  about  8  cm.  Plot  a  curve 
with  distances  as  ordinates  and  current  as  abscissae.  Observe 
the  different  straight  line  sections  of  the  curve  showing  the 
complex  nature  of  the  source. 

Heat  a  specimen  of  this  radium  bromide  to  drive  off  its 
emanation  and  other  products  and  then  redetermine  the  curve. 
Observe  that  it  is  of  a  much  simpler  character  than  in  the 
former  instance,  showing  that  the  source  is  less  complex. 

Expose  a  thin  copper  wire  in  radium  emanation  for  a  couple 
of  hours  to  obtain  an  active  deposit  upon  it.  Remove  the  wire 
and  use  the  radium  C  as  the  source  of  radiation  as  described 
in  §  149.  Using  the  steady  deflection  method  determine  an 


228 


ANALYSIS    BY   ABSORPTION    CURVES 


experimental  curve  for  current  and  distance  for  the  radium  C. 
The  curve  should  take  the  form  shown  in  Fig.  75,  which  is  a 
specimen  curve  obtained  by  actual  experiment  in  an  investiga- 
tion by  the  author.  Since  the  radium  C  decays  during  the  time 
of  the  observations,  the  readings  must  be  corrected  for  the 

decay  of  the  source.  This 
may  be  done  by  bringing 
the  source  R  back  to  a 
fixed  position  after  about 
every  four  or  five  readings 
and  measuring  the  current 
for  that  position,  and  at  the 
same  time  observing  the 
time  intervals.  The  rate 
of  decay  can  thus  be  ob- 
served and  the  necessary 
correction  made  for  a  con- 
stant source. 

If  a  sample  of  radiotho- 
rium  is  available  determine 
a  corresponding  curve, 
using  it  as  the  source  of 
rays.  It  will  be  found  to 


FIG.  75. 


consist  of  two  distinct 
parts,  showing  the  com- 
plexity of  the  source.  It  was  by  a  determination  of  this  very 
curve  for  radiothorium  that  Hahn  discovered  the  previously 
unknown  or  unsuspected  substance,  thorium  C. 

These  experiments  all  show  that  the  a  particles  emitted  by 
different  radio-active  substances  have  definite  ranges  over 
which  they  will  produce  ionization  and  that  the  ionization  stops 
abruptly  at  given  distances  from  the  source,  these  distances 
depending  upon  the  velocity  of  initial  projection. 

Instead  of  detecting  the  a  rays  by  the  ionization  produced, 
their  presence  may  be  detected  by  the  scintillations  produced 
on  a  zinc  sulphide  screen.  Their  range  of  action  should  be 
determined  by  this  method  and  it  will  be  found  to  be  the  same 
as  their  range  of  activity  as  ionizers. 


CHAPTER  XVI. 
RADIO-ACTIVITY    OF    THE   ATMOSPHERE. 

152.  Loss  of  Charge  in  Closed  Vessels. — Make  an  electro- 
scope of  the  form  shown  in  Fig.  14  of  about  500  c.c.  in  volume. 
For  this  purpose  a  perfectly  new  one  should  be  made,  and  do 
not  under  any  circumstances  use  one  that  has  been  connected 
in  any  way  whatever  with  other  radio-active  measurements. 
In  addition,  every  portion  of  the  material  used  in  its  manufac- 
ture should  be  absolutely  free  from  the  slightest  chance  of  pre- 
vious contamination  by  any  radio-active  body,  and  all  the 
measurements  which  are  to  follow  should  be  made  in  a  labo- 
ratory where  no  radio-active  material  has  ever  been  used. 

For  this  purpose  it  will  be  found  convenient  to  select  a  brass 
tube  about  8  cm.  diameter  and  10  cm.  long  for  the  containing 
vessel.  Close  the  ends  by  brass  plates  so  as  to  be  air-tight. 
Thoroughly  clean  every  portion  of  the  surface  of  the  brass 
with  emery  paper.  Arrange  the  insulated  system  as  in  Fig. 
14.  Instead  of  the  charging  arrangement  F,  attach  a  very 
flexible  piece  of  steel  watch  spring  to  the  lower  end  of  the 
rod  D,  as  shown  in  (a),  Fig.  14,  and  bend  it  into  a  convenient 
shape  so  that  it  will  not  touch  the  sulphur  head  and  will  also 
hang  clear  of  the  lower  rod  H,  but  so  that  the  lower  end  of  the 
spring  may  be  brought  temporarily  in  contact  with  H ,  either  by 
bringing  a  strong  magnet  near  it  or  by  tilting  the  whole  elec- 
troscope and  slightly  jarring  it.  This  arrangement  will  allow 
the  instrument  to  be  made  gas-tight. 

Set  this  electroscope  up  in  a  room  perfectly  free  from  the 
slightest  contamination  from  any  radio-active  material  and  fill 
it  with  perfectly  dry  and  dust-free  air.  Charge  up  the  gold 
leaf  system  to  about  200  volts  and  observe  any  slow  movement 
of  the  gold  leaf  in  the  usual  way  by  a  reading  microscope  with 
micrometer  eye-piece.  During  these  readings  keep  the  rod  D 
permanently  connected  to  a  battery  of  200  volts. 

229 


230  RADIO-ACTIVITY   OF   THE   ATMOSPHERE 

Observe  that  the  potential  of  the  gold  leaf  system  slowly 
decreases.  This  decrease  cannot  be  due  to  imperfect  insula- 
tion of  the  sulphur  bead,  for  since  D  is  maintained  at  a  con- 
stant potential  as  high  or  higher  than  the  gold  leaf  any  leakage 
across  the  sulphur  would  take  place  from  D  to  the  gold  leaf 
instead  of  the  other  way  and  tend  to  maintain  the  potential  of 
the  gold  leaf  instead  of  allowing  it  to  decrease.  The  leakage 
must  then  be  through  the  air  and  must  therefore  be  due  to  the 
presence  of  ions  in  the  air. 

Determine  the  electrostatic  capacity  of  the  electroscope  and 
measure  the  current  per  second  corresponding  to  this  slow 
leak,  and,  assuming  the  charge  on  the  ion  (§81),  calculate  the 
number  of  ions  produced  in  the  air  per  second  in  the  volume 
contained  in  the  electroscope  and  then  determine  the  number 
per  cubic  centimeter. 

A  great  variety  of  experiments  of  this  nature  have  been 
performed  by  a  number  of  observers  and  it  is  found  that  there 
are  always  a  few  ions  present  in  the  air  or  any  other  gas,  but 
the  number  varies  somewhat  under  different  conditions.  Gases 
thus  always  possess  at  least  a  small  amount  of  conductivity. 
Every  electroscope  has  therefore  what  is  usually  termed  its 
natural  rate  of  leak,  and  in  using  the  electroscope  for  making 
radio-active  measurements  this  natural  rate  of  leak  must 
always  be  independently  determined  and  subtracted  from  the 
total  rate  of  leak. 

153.  Effect  of  Conditions  on  the  Natural  Leak. — Measure 
this  natural  rate  of  leak  for  the  air  at  different  pressures  in  the 
electroscope.  Note  that  it  is  approximately  proportional  to  the 
pressure.  Measure  also  this  natural  conductivity  for  various 
gases  at  the  same  pressure  and  note  that  the  greater  the  density 
the  greater  the  conductivity,  although  it  is  not  quite  propor- 
tional in  all  cases. 

If  the  electroscope  be  surrounded  underneath  and  on  all  sides 
by  lead  sheets  about  5  cm.  thick  the  natural  rate  of  leak  will 
be  diminished  by  quite  a  large  percentage,  but  a  further  increase 
in  thickness  of  the  lead  will  cause  no  further  diminution  in  the 
conductivity.  This  indicates  that  part  at  least  of  the  natural 


EXCITED   ACTIVITY    FROM    THE   AIR  23! 

conductivity  is  due  to  a  very  penetrating  radiation  of  some  sort 
from  the  earth  and  air. 

154.  Excited  Activity  in  the  Atmosphere. — Suspend  a  cop- 
per wire,  12  or  14  meters  long,  in  the  open  air  and  keep  it 
charged  to  a  high  negative  potential  of  about  10,000  volts  for 
an  hour  or  two  by  connecting  it  to  the  negative  pole  of  a  Wims- 
hurst  machine.  Then  remove  it  and  quickly  wind  it  back  and 
forth  on  an  open  rectangular  framework  about  80  cm.  long  and 
20  cm.  wide.  Suspend  and  insulate  this  frame  in  the  centre  of 
a  metal  cylinder  of  about  25  cm.  diameter  and  100  cm.  long. 
Test  in  the  usual  manner  by  means  of  a  sensitive  electrometer, 
for  an  ionization  current  between  the  cylinder  and  this  wire 
acting  as  the  central  electrode.  The  wire  will  be  found  to  be 
radio-active  and  to  act  in  all  respects  like  a  wire  made  active  by 
exposure  to  an  emanation.  The  activity  of  the  wire  can  be 
partially  removed  by  rubbing  and  by  dipping  the  wire  in  acid 
solution.  It  also  decays  with  time,  falling  to  half  value  in  about 
three  quarters  of  an  hour.  Obtain  a  decay  curve  for  it  in  the 
usual  manner. 

Since  under  no  other  circumstances  has  an  active  deposit 
of  this  nature  been  obtained  except  in  the  presence  of  an  ema- 
nation it  appears  from  these  results  that  there  must  be  an 
emanation  of  some  sort  in  the  atmosphere.  This  question 
has  been  investigated  by  different  observers  using  different 
methods,  and  it  has  been  shown  that  both  radium  and  thorium 
emanations  are  present  in  the  atmosphere.  The  amount  of 
these  emanations  in  the  atmosphere  is  different  in  different 
localities.  The  excited  activity  obtained  on  a  negatively 
charged  wire  in  the  air  is  due  to  a  mixture  of  the  active 
deposits  from  both  radium  and  thorium,  but  on  account  of  the 
very  rapid  decay  of  thorium  emanation  most  of  the  activity 
arises  from  the  radium  emanation.  It  is  not  surprising  that 
these  emanations  should  be  found  in  the  atmosphere,  for  since 
both  radium  and  thorium  are  known  to  be  distributed  among 
different  deposits  throughout  the  earth  the  emanations  must 
be  continually  diffusing  into  the  atmosphere  in  small  quantities. 
The  presence  of  this  radio-active  matter  in  the  air  will  explain 


232  RADIO-ACTIVITY  OF  THE  ATMOSPHERE 

to  some  extent  at  least  the  natural  conductivity  of  the 
atmosphere. 

If  freshly  fallen  snow  or  rain  be  collected  and  quickly  evapo- 
rated to  dryness  in  a  platinum  vessel  the  residue  will  be  found 
to  be  radio-active  if  tested  by  an  electroscope.  This  active 
residue  acts  in  a  manner  similar  to  that  of  the  active  deposit 
obtained  on  the  negatively  charged  wire  exposed  to  the  atmos- 
phere. The  rate  of  decay  of  the  active  residue  from  snow 
or  rain  is  a  little  more  rapid  than  the  rate  of  decay  of  the 
active  deposit  on  the  wire.  The  falling  rain  or  snow  appar- 
ently carries  down  this  active  matter  from  the  atmosphere  and 
it  remains  behind  after  evaporation  of  the  rain  or  snow. 

Meteorological  conditions  are  observed  to  influence  the  amount 
of  excited  activity  obtainable  from  the  atmosphere  to  a  con- 
siderable extent.  The  amount  of  excited  activity  is  subject 
to  great  variations.  The  activity  is  greater  at  the  tempera- 
tures below  o°  C.  than  at  those  above  o°  C.  The  lowering  of 
the  barometric  pressure  causes  an  increase  in  the  amount  of 
excited  activity. 

155.  Present  State  of  the  Subject  of  Atmospheric  loniza- 
tion. — The  whole  subject  of  the  radio-activity  of  the  atmos- 
phere is  a  wide  one,  as  there  is  such  a  variety  of  existing  con- 
ditions to  be  taken  into  account.  It  is  influenced  by  meteoro- 
logical conditions,  by  the  presence  of  any  radio-active  de- 
posits in  the  neighborhood  and  by  particular  local  conditions. 
There  are  also  different  kinds  of  radiations  which  unite  to 
produce  this  conductivity  of  the  air,  some  of  which  are  very 
easily  absorbed  and  others  of  which  are  extremely  penetrating. 
The  conductivity  is  so  complex  and  also  subject  to  such  fluctu- 
ations that  it  is  difficult  to  obtain  perfectly  definite  results  on 
which  a  satisfactory  general  explanation  may  be  based.  A 
great  deal  of  work  has  been  done  on  this  subject,  but  there  still 
remains  a  considerable  amount  of  very  careful  investigation  to 
be  carried  out  in  order  to  place  the  subject  on  a  satisfactory 
basis. 


INDEX. 


The  numbers  refer  to   the  pages. 


a  rays 

from  uranium,  141 

nature  of,   141,  180 

absorption    of,    by    solids,    141, 

143,  160,  162 
penetrating  power  of,   141,   143, 

160,   162 

from  thorium,  144 
from  radium,  144 
magnetic  deflection  of,  147 
sign  of  charge  carried  by,   150, 

151 

electrostatic  deflection  of,   151 
from   polonium,    152 
relative   ionization   by,    154 
photographic  action  of,  156 
phosphorescent   action   of,    157 
scintillations  by,   158 
complexity     of,     from     radium, 

159 

separate  source  of,   160 
absorption  of,  by  gases,   165 
effect    of    pressure    on    absorp- 
tion of,  by  gases,   168 
effect   of   absorption   on   ioniza- 
tion by,   1 68,   170 
charge  carried  by,  174,  180 
number  of  a  particles   emitted, 

175 

velocity  of,  177,  180 
mass  of,  1 80 

a  particle  as   helium   atom,    181 
energy  of,    181 

from    thorium    emanation,    193 
from   radium   emanation,    197 
from  actinium  emanation,   198 


a  rays 

active  deposit  from  radium,  213 
homogeneous  source  of,  223 
absorption   of,   from  radium    C, 

227 
Absorption 

of    Rontgen    rays,    55,    59,    79 

et  seq. 

law   of,   82,    1 60,    1 66 
coefficient  of,  82,   160,   166 
of  uranium  rays  by  solids,  141, 

159  et  seq. 
of   a,  /3   and  y  rays   by   solids, 

141,    159,   161 
of  rays  from  radium,   143,   160, 

161,    164 

of  rays  from  thorium,  143 
effect  of  thickness  of  layer  on, 

161   et  seq. 
of    rays    by    gases,    82,    164    et 

seq. 

effect  of  pressure  on,  168 
effect    of,    on    ionization,     167, 

1 68  et  seq.,  170 
of  a  rays  as  method  of  analy- 
sis,  224  et  seq. 
Accumulators,  7 
Actinium 

discovery   of,    135 

activity  of,  135 

recovery   of   activity   of,    186 

emanation    from,    197 

excited   radio-activity   from,  216 

transformation  products  of,  216, 

217 


233 


234 


INDEX. 


Actinium   X 

chemical  separation  of,  186 

decay  of  activity  of,  186 

as  source  of  emanation,  204 
Air-tight  joints,  38 
Amalgamation 

of  mercury  contacts,  21 
Amber 

use  of,  in  electrometer,  14 

as  insulation,  18 
Analysis 

by   absorption   curves,   223,   224 

et  seq. 
Anode 

effect    of    position    of,    on    dis- 
charge, 5 
Apparatus 

descriptions  of,  7  et  seq. 
Atoms 

theory  of  structure  of,  219 

disintegration  of,  219 

unstable  nature  of,  219 

/8  rays 

from  uranium,   142 
nature  of,  142,  145  et  seq. 
absorption    of,    by    solids,    141, 

143,  160,  163 
penetrating  power  of,   141,   160, 

163,  164 

from  thorium,  144 
from  radium,  144 
magnetic  deflection  of,   145 
sign  of  charge  carried  by,   146, 

150 

electrostatic  deflection  of,  150 
relative    ionization    by,    154    et 

seq. 

photographic  action  of,  156 
phosphorescent  action  of,  157 
complexity     of,     from     radium, 

159,  176 
absorption    of,    by    gases,     164, 

167 


|8  rays 

effect  of  pressure  on  absorption 
of,    170 

effect   of   absorption   on   ioniza- 
tion  by,    171 

charge  carried  by,   172 

number  of  /8  particles   emitted, 
173 

velocity  of,   176 

value  of  e/m  for,  176 

variation    of    e/m    with    speed, 
176 

apparent  mass  of,  176 

loss  of,  by  uranium,  184 

from  Ur.  X,  184 

recovery  of,  by  uranium,  184 

decay  of,  from  Ur.  X,  184 

slow    moving,    emitted    by    tho- 
rium A,  211 

emitted  by  active  deposit  from 

radium,  213 
Barium  platinocyanide 

phosphorescent      action      under 

rays,  158 
Becquerel 

discovery    of    rays    from    ura- 
nium, 127 

velocity  of  /3  rays,    176 

value  of  e/m  for  j8  rays,  176 

separation  and  study  of  Ur.  X, 

183 
Bragg 

theory  of  y  rays,  182 
Bronson 

steady     deflection     method     of 

measuring  currents,  136 
Brush   discharge,    i 

Calibration 

(see  standardization) 
Canal  rays 

discovery  of,   51 

deflection  of,  51 

velocity  of,  51 

value  of  e/m  for,  51 


INDEX. 


235 


Capacity 

of  electrometer,  24 

methods    of    determination    of, 

24 

of   Dolazalek   electrometer,   27 
of  electroscope,  31 
of   condensers,   33 
Carbon 

ionization   from   heated,    114 
Cathode    rays 

production  of,   6 
phosphorescence     produced    by, 

6,   40 

opacity  of  solid  bodies  to,  40 
heating  effect  of,  41 
magnetic  deflection  of,  41 
electrostatic  deflection  of,  42 
negative  charge  carried  by,  42, 

43 

velocity  of,  44  et  seq. 
value  of  elm  for,  44  et  seq. 
mass   of   cathode  particle,   49 
Charcoal 

use  of  to  produce  vacuum,  36 
Charge 

ratio    of    charge    to    mass    for 

cathode  particle,  44  et  seq. 
ratio    of    charge    to    mass    for 

electrolytic  ion,  48 
carried  by  gaseous  ion,  49,  124, 

1 80 
ratio    of    charge    to    mass    for 

Lenard  rays,  50 
ratio    of    charge    to    mass    for 

canal  rays,  51 
ratio    of    charge    to    mass    for 

corpuscle   from   heated   solid, 

in 
sign  of,  carried  by  (3  rays,  146 

151 
sign  of,  carried  by  a  rays,  150, 

151 

carried  by  j3  particle,  172,  174 

carried  by  a  particle,  174,  180 

effect  of  charge  in  motion,  176 


Charge 

ratio  of  charge  to   mass   for  /3 

rays,   176 
ratio   of  charge  to   mass   for  a 

rays,  177  et  seq. 
carried  by  hydrogen  atom,   180 
loss  of,  in  closed  vessels,  229 
"  Glock  " 

"radium,"   173 
Clouds 

production     of,    by     expansion, 

1 1 8,  120  et  seq. 
production    of,    around    ions    as 

nuclei,  118,  121,  123 
Coefficient 

of  absorption  of  gases,  82 
of  recombination  of  ions,   89 
of  diffusion  of  ions,  96 
Collision 

ionization  by,   104  et  seq. 
explanation  of  electric  spark  by, 

1 06 
Condensation 

of    water    vapor    around    ions, 

118  et  seq. 
of  emanations,  200 
Condenser 

in  quadrant  electrometer,  10,  n 
different  forms  of,  33 
Conductivity 

of    gases    exposed    to    Rontgen 

rays,  60  et  seq. 
produced    by    ultra-violet    light, 

107,  109 
produced  by  incandescent  solids. 

in  et  seq. 
of  flames,  115 
produced  by  uranium,  129  et 

seq. 

produced  by  polonium,   134 
produced  by  radium,  134  et  seq. 
produced  by  actinium,  135 
of  the  atmosphere,  229  et  seq. 
Corpuscle,  49 


236 


INDEX. 


Corpuscles 

liberation     of,     by     ultra-violet 
light,   107 

escape   of,   from   heated  metals, 

in  et  seq. 
Crookes 

dark  space,  4 

discovery  of  Ur.  X,  183 
Curie,  Mme. 

examination       of       radio-active 

minerals,  133 
Curie,  M.  et  Mme. 

discovery  of  polonium,   134 

discovery  of  radium,   134 

charge  carried  by  ft  rays,  172 
Current 

measurement    of,    by    electrom- 
eter, 26,   70,  76,   136 

measurement     of,     by     electro- 
scope,  31 

produced  by  X   rays  in  air,   70 
et  seq. 

variation  of,  with  voltage,  72 

saturation,  73 

variation   of,   with   distance  be- 
tween plates,  73,  170 

explanation  of,  74 

absolute  measure  of,  76 

dependence    of,    on    quality    of 
rays,    78 

produced  by  uranium  rays,   130 
et  seq. 

produced   by    other    radio-active 
bodies,  135 

Dark  space 

Crookes,  4 
Faraday,   4 

Decay 

of  activity  of  polonium,  134 
of  activity  of  Ur.  X,  183,  184 
of  activity  of  Th.  X,  186 
of  activity  of  Act.  X,  186 
of   activity   of   thorium   emana- 
tion, 194 


Decay 

of    activity    of    radium    emana- 
tion,  196 

of  activity   of  actinium   emana- 
tion, 197 

of   emanation    at    low    tempera- 
ture, 202 

of    excited    activity    from    tho- 
rium, 208  et  seq. 

of  excited  activity  from  radium, 
211   et  seq. 

of  excited  activity  from  atmos- 
phere, 231 
De-emanation 

of  thoria,  199 

of  radium  compounds,  199 
Deflection 

of  cathode   rays,   41,  42 

of  j8  rays,  145,  150 

of   a   rays,    147,    151 

as  method  of  differentiation,  153 
Deposit,  active,  205  et  seq. 

from   thorium,    radium   and   ac- 
tinium, 205 

concentration  on  negative  elec- 
trode, 206 

source  of,  207 

effect  of  dust  on,  209 

analysis   of,   from  thorium,   210 

method   of   deposition   of,   from 
radium,  211 

analysis  of  from  radium,  214  et 
seq. 

from  actinium,  216 

general  properties  of,  216 

phosphorescent  action  of,  216 

from  the  atmosphere,   231 
Differentiation 

methods  of,  of  rays,   153 

between     ionizing     and     photo- 
graphic effects  of  rays,  153 
Diffusion 

of  ions,  93  et  seq. 
Discharge 

brush,  i 


INDEX. 


237 


Discharge 

spark,  2 

effect  of  pressure  on,  3 
Disintegration 

theory  of,  218  et  seq. 
Disturbances 

electrostatic,  17 

remedy  for,  17 
Dolazalek 

electrometer,  14 

capacity  of  electrometer,  27 

Earthing 

of  electrometer  quadrants,  16,  21 

of  screens  and  connections,   17 

key,  20 

of  electroscope,  29 
Ebonite 

as  insulator,  18 
Electric  field 

action  on  ionized  gas,  63 
Electrometer,  quadrant 

description  of,  9  et  seq. 

sensitiveness  of,    n,  23 

suspension  of  needle,  9,  n,  12 

charging  of  needle,   12 

potential  of  needle,  10,  12 

theory  of,  12 

Dolazalek  type,  14 

lamp  and  scale  of,  15 

adjustment  and  testing  of,  16 

insulation  of,  16 

screening  of,  17 

special  keys  for,  20 

standardization  of,  22 

connection    of,    to    other    appa- 
ratus,  23 

capacity  of,  24,  27 

use  of  to  measure  current,   26, 
70  et  seq.,  76 

experiments   with,   27 

steady  deflection  method,   136 
Electron,  49 
Electrons 

(see  corpuscles) 


Electroscope 

description   of  general  type,  27 
et  seq. 

charging  of,  29,  229 

illumination  of  scale,  30 

method  of  use  of,  30 

calibration  of,  31 

capacity  of,  31 

use  of  to  measure  current,  31 

experiments  with,  32 

natural  leak  of,  32,  230 

use  of,  with  X  rays,  60  et  seq. 
Emanating  power 

of   thorium   compounds,    198   et 
seq. 

effect  of  conditions   on,   198   et 
seq. 

partial  destruction  of,   199 

restoration  of,  199 

relation   to    active   deposit,    208 
Emanation 

discovery  of,  from  thorium,  190 

properties  of,  from  thorium,  190 

gaseous  nature  of,   192 

diffusion  of,  through  solids,  192 

decay  of,  from  thorium,  194 

continuous  emission  of,   196 

from  radium,  196 

from  actinium,   197 

effect  of  conditions  on  emission 
of,  198 

condensation  of,  200 

temperature  of  volatilization  of, 
201,  202 

rate   of   decay   of,   at   low  tem- 
peratures,  202 

phosphorescent  action  of,  202 

source  of,  203  et  seq. 

as     source     of     excited     radio- 
activity, 206,  207 

as  source  of  thorium  A,  210 

in  the  atmosphere,  231 
Energy 

absorption    of,     of    a    rays     in 
gases,  167 


INDEX. 


Energy 

of  a  particle,  181 

radiation  of,  by  complex  atom, 

220 
Excited  radio-activity 

observation  of,  205  et  seq. 

concentration  on  negative  elec- 
trode, 206 

source  of,  207 

decay  of,  from  thorium,  208  et 
seq. 

rise  of,  from  thorium,  208 

effect  of  dust  on,  209 

explanation     of     decay     curves, 
210 

from  radium,  211  et  seq. 

from  actinium,  216 

in  the  atmosphere,  231 
Expansion 

production    of    clouds    by,    118, 
120  et  seq. 

apparatus  for,   118 

Faraday  dark  space,  4 
Fatigue 

phosphorescent,   due  to  cathode 
rays,  41 

photo-electric,   1 1 1 
Flames 

conductivity  of,  115  et  seq. 

discharge  of  electrified  body  by, 

19,  116 

Fleuss  pump,  35 
Focus  tube 

simple  form  of,  53 

setting  up  of,  58,  67 

automatic  type,  64 

manipulation  and  care  of,  67 

y  rays 

from  uranium,  143 
nature  of,  143,  181 
penetrating  power  of,  143,  144, 

161,  163,  164 
from  thorium,  144 


7  rays 

from  radium,  144 
non-deviability  of,  151,  181 
relative  ionization  by,  154 
photographic  action  of,  156 
phosphorescent  action  of,  158 
absorption  of  by  gases,  16^  167 
origin  of,  182 

Geiger  and  Rutherford 

(see  Rutherford  and  Geiger) 

Glow 

negative,  4 

Goldstein 

discovery  of  canal  rays,  51 

Guard-ring,  77 

Hahn 

separation  of  thorium  D,  211 
transformation  products  of  tho- 
rium, 217 

discovery  of  thorium  C,  228 
"  Hard  "  and  "  Soft  "  X  rays,  55,  64, 

78 
Helium 

identity  of   a  particle  with  he- 
lium  atom,    181 

found   in   old   radio-active  min- 
erals,  181 
Hertz 

penetration    of    solid    body    by 

cathode  rays,  49 
Homogeneous  source 
of  a  rays,  223 

Illumination 

of  electroscope  scale,  30 

Incandescent  solids 

ionization  by,  in,  112,  114 
value  of  e/m  for  ions  from,  in 

Inertia 

apparent  inertia  due  to  motion, 
176 

Insulation 

test  of,  in  electrometer,  16 
different  materials  used  for,   18 


INDEX. 


239 


Insulation 

precautions  in  regard  to,  19,  20 
charging  of  surface  of,  19 
location  of  defective,  19 
Ionium 

parent  of  radium,  218,  222 
lonization 

by  Rontgen  rays,  60  et  seq. 

theory  of,  74 

effect  of  quality  of  rays  on,  64, 

78 
effect  of  distance  between  plates 

on,   73,   131 
effect    of    pressure    of    gas    on, 

85,  168 
effect  of  nature  of  gas  on,  87, 

132 

by  collision,  104 
by  cathode  rays,  105 
by  ultra-violet  light,  108,  109 
by    incandescent   solids,    in    et 

seq. 

by  flames,  115  et  seq. 
by  uranium  rays,   129  et  seq. 
effect   of  thickness   of  material 

on,  131 
by     other     radio-active    bodies, 

133,  135 

measurement   of   by   steady   de- 
flection,   136 

as  method  of  differentiation,  153 
comparison  of,   for  a,  /3  and  y 

rays,   153 
as  method  of  test  compared  with 

photographic  method,  156,  184 
proportional  to   absorption,    167 
in  the  atmosphere,  230  et  seq. 
Ions 

negative  ion,  49 
positive  ion,  51 
in  explanation  of  conductivity 

of  gases,  74  et  seq. 
recombination  of,  88  et  seq. 
diffusion  of,  93  et  seq. 


Ions 

relation    of    negative    ion    and 

electron,  95 

mobility  of,  98  et  seq. 
by  collision,    104 
by  ultra-violet  light,  107,  109 
from    incandescent    solids,    in 

et  seq. 

from  flames,  115 
as  nuclei,  118  et  seq. 
charge  carried  by,   124 
number  of,  in  air,  230 

Kaufmann 

relation    between    velocity    and 

e/m  for  j8  rays,  176 
Keys 

special  for  electrometer  use,  20 

Langevin 

recombination  of  ions,  91 
Lead 

as  screen  for  Rontgen  rays,  57 
Lenard  rays,  49  et  seq. 

Marx 

velocity  of  X  rays,  53 
Mass   (see  also  charge) 

of  cathode  ray  particle,  49 

apparent  mass  of  j9  particle,  176 

due   to    electric   charge    in   mo- 
tion, 176 

of  a  particle,  180 
Measurement 

general  methods  of,  for  X  rays, 

69 
Mercury   cups 

for  electrometer  connections,  21 
Mercury    interrupter 

use  of,  with  X  ray  bulb,  68 
Mesothorium,  217 
Metabolon,  220 
Microscope 

use  of,  with  electroscope,  30 


240 


INDEX. 


Mobility  of  ions,  98 
Moisture 

effect  on  insulation,  19 

effect   on  mobility   of   ions,   99, 

103 

effect  on  emission  of  emanation, 
198 

Natural  leak 

of  electroscope,  32,  230 

in  closed  vessels,  229  et  seq. 

effect  of  conditions  on,  230 

Needle 

of  electrometer,  9 
potential  of,  9,  10,   12,   15 

Negative  glow,   4 

Nuclei 

dust  particles  as,  118,  121 
ions  as,  118,  121  et  seq. 

Number 

of  ft  particles  emitted,  173 
of  a  particles  emitted,  175 
of  ions  in  the  atmosphere,  230 

Occlusion 

of     gas     in     electric     discharge 

tubes,  4,  37 
of  emanation  by  solids,   199 

Paraffin 

as  insulator,  19 

use  of  as  mercury  cup,  22 

condenser,  34 

Penetrating  power 

of  Rontgen  rays,  55  et  seq.,  64, 

79  et  seq. 
of   a,   )8   and  7   rays,    141,    143, 

159  et  seq. 

as    method     of    differentiation, 
153 

Period 

of  recovery  of  uranium,  185 
of  decay  of  Ur.  X,   185 
of  recovery  of  thorium,  186 
of  decay  of  Th.  X,   186 
of  recovery  of  actinium,  186 


Period 

of  decay  of  Act.  X,  186 
definition    of,    189 
of    decay    of    thorium    emana- 
tion, 195 

of  rise  of  thorium  activity,  196 
of  decay  of  radium  emanation, 

197 
of  decay  of  actinium  emanation, 

198 
of   decay   of   emanation   at   low 

temperature,  202 
of     decay     of     excited     activity 

from  thorium,  209 
of    transformation    of    thorium 

A  and  B,  211 
of  transformation  of  radium  A, 

B  and  C,  214 
of  transformation  of  radium  D, 

E,  F  and  G,  216 
of    transformation    of    actinium 

A,  B  and  C,  216 
of     transformation     of     radio- 
thorium,  217 
of     transformation     of     meso- 

thorium,    217 
Perrin 

charge  carried  by  cathode  rays, 

43 
Phosphor-bronze 

as  electrometer  suspension,  12 
Phosphorescence 

due  to  cathode  rays,  6,  40 
due  to  Rontgen  rays,  53,  55 
of  uranium  salts,  127 
as     method     of     differentiation, 

153 

by  different  types  of  rays,  157 
of  emanations,  202 
Phosphorescent 

screen  for  use  with  X  rays,  55 
Photo-electric 

effect,  107,  109 
fatigue,   iii 


INDEX. 


241 


Photographic  action 

of  Rontgen  rays,  59 

of  rays  from  uranium,  128 

of  rays  from  thorium,  133 

as  means  of  differentiation,  153 

test    compared    with    ionization 

test,  156,  184 
of  Ur.  X,  184 

Pitch-blende 

activity  of,  133 
as  source  of  polonium,  134 
as  source  of  radium,  134 
as  source  of  actinium,  135 

Platinum 

ionization  from  heated,  112 

Polonium 

discovery  of,   134 

decay   of  activity   of,    134 

nature  of  radiations  from,   134, 

152 

devoid    of    emanation    and    ac- 
tive deposit,  208 

Positive    column 
definition  of,  4 
change  in  length  of,  5 

Potential 

to  produce  spark,   i,  2 
of  electrometer  needle,  9,  10,  12, 
IS 

Pressure 

effect  on  electric  discharge,  3 
effect  on  ionization,  85,  168 
effect  on   absorption  of  a  rays, 

168 

effect    on    emission    of    emana- 
tion,   198 

Pumps,  35 

Quadrant     electrometer     (see     elec- 
trometer) 

Quartz 

as  electrometer  suspension,   n 
use    of,    with   ultra-violet   light, 
1 08 


Radiations 

emitted  by  uranium,  127  et  seq., 

141 
photographic  action  of,  127,  128, 

156 

discharging  power  of,  129 
emitted  by  thorium,  133,   144 
emitted  by  polonium,  134,  152 
emitted  by  radium,  134,  144 
emitted  by  actinium,  135 
complexity  of,  141  et  seq.,  159 
different  types  of,  143 
absorption   of,   90   et  seq.,   159, 

162  et  seq. 

deflection  of,  145  et  seq. 
general  properties  of,  153  et  seq. 
methods  of  differentiating,  153 
relative  ionization  by,  153 
complexity    of    /3    and    a    rays 

from   radium,   159 
special  constants  of,  172  et  seq. 
emitted  by  Ur.  X,  184 
emitted  by   thorium   emanation, 

193 
emitted    by    radium    emanation, 

197 
emitted  by  actinium  emanation, 

198 
emitted    by    thorium    products, 

211 
emitted    by    active    deposit    of 

radium,  214,  216 
emitted    by    active    deposit    of 

actinium,  216 

emitted  by  radio-thorium,  217 
emitted  by  mesothorium,  217 
emitted  by  radio-actinium,  218 
Radio-actinium,  218 
Radio-active   changes 

theory  of,  218 
Radio-active  elements 

table  of,  222 
Radio-activity 

discovery  of,   127 
application  of  term,  133 


242 


INDEX. 


Radio-activity 

accompaniment     of     disintegra- 
tion, 219 

of  the  atmosphere,  229  et  seq. 
of  snow  and  rain,  232 
Radiothorium 

radiations  emitted  by,  217 
analysis    of   by    method    of    ab- 
sorption curves,  228 
Radiouranium,  218 
Radium 

discovery  of,   133 

activity  of,  134 

quantity     of,     in     pitch-blende, 

134 

source  of,  134 
complexity    of    rays    from,    144 

et  seq. 
deflection  of  j8  rays  from,   145, 

150 
deflection  of  a  rays  from,   147, 

151 
relative  ionization  by  rays  from, 

154 
photographic     action     of     rays 

from,  157 
phosphorescent    action    of    rays 

from,   158 

complexity  of  0  rays  from,   159 
complexity  of  a  rays  from,  159 
"  clock,"  173 
theory  of  successive  changes  in, 

188 

emanation,   196 

rise  of  activity  with  time,    197 
emanating  power  of,   198 
de-emanation  of,  199 
as  source  of  emanation,  204 
excited   radio-activity   from,  205 

et  seq. 
decay  of  excited  activity  from, 

211    et  seq. 

transformation  products  of  rapid 
change,    214 


Radium 

transformation  products  of  slow 
change,  215 

relation  to  uranium,  218 

parent  of,  218 

structure  of  atom  of,  219 
Radium  A,   214 
Radium   B,   214 
Radium  C 

as  source  of  a  rays,  177,  223 

rays  emitted  by,  214 

absorption  of  a  rays  from,  227 
Radium  D,  215 
Radium  £,215 
Radium  F,  215 
Radium  G,  215 
Rain 

radio-activity  of,  232 
Rayless  products,  216,  217 
Rayless  changes 

discussion  of,  220 
Recombination 

of  ions,  88  et  seq. 
Rontgen 

discovery  of  rays.  53 
Rontgen  rays 

origin  of,   53 

discovery   of,    53 

velocity  of,  53 

production  of,  by  focus  tube,  41 

phosphorescent  action  of,  53,  55 

penetrating  power  of,  55  et  seq., 
59 

"hard"    and    "  soft "    rays,    55, 
64,  78 

absorption  of,  56  et  seq.,  59,  79 

use  of  lead  as  screen  from,  57 

photographic  action  of,  59 

conductivity  produced  by,  60  et 
seq. 

quantitative    measurements    on, 
64  et  seq. 

current  in  air  produced  by,   70 

effect  of  quality  of,  78 


INDEX. 


243 


Rutherford 

deflection  of  a  rays,   147 

effect  of  pressure  on  ionization, 
169 

curves  showing  effect  of  dis- 
tance between  plates  on  cur- 
rent, 171 

velocity  and  elm  for  a  rays, 
177 

charge  carried  by  gaseous  ion, 
1 80 

discovery  of  emanation,  190 

rays  emitted  by  emanation,  193 
Rutherford  and  Geiger 

charge    carried    by    a    particle, 

174 
number   of  a  particles  emitted, 

174,  175 
Rutherford  and  McClung 

absorption  of  X  rays  by  gases, 

83 
Rutherford  and  Soddy 

separation  of  Th.  X,  183 
decay  and   recovery   curves   for 

uranium  and  Ur.  X,  185 
theory    of    successive    changes, 

188,  218 

condensation  of  emanations,  200 
Rutherford  and  Thomson 
theory  of  ionization,  74 

Saturation 

curve,   73 

current,  73 

explanation  of,  75 

curve  for  uranium  rays,  131 
Scintillations 

by  a  rays,  158 

used  to  count  a  particles,  175 

used    to    detect    a    rays    from 

radium   C,  228 
Screening 

of  electrometer,  17 

of  wire  connections,   18 


Screening 

of  apparatus  from  Rontgen  rays, 

57 
Screens 

phosphorescent,  55 

lead  as  protection  from  X  rays, 
57 

to  limit  beam  of  X  rays,  57 
Snow 

radio-activity  of,  232 
Soddy  and  Rutherford  (see  Ruther- 
ford and  Soddy) 
"  Soft "    and    "  hard "    X    rays,    55, 

64,  78 
Spark,  electric 

discharge,   2 

relation  of,  to  potential,  2 

explanation  of,  106 
Standard 

test  ionization  apparatus,  80 
Standard  cell 

use  of  with  electrometer,  22 
Standardization 

of  electrometer,  22 

of  electroscope,  31 

of    condenser,    34 
Steady   deflection 

description    of   method   of,    136 
Strise,  4 
Strutt 

electrometer   condenser,    n 

"radium  clock,"  173 
Successive  changes 

theory  of,  187,  218 
Sulphur 

as  insulator,  18 

precautions  in  regard  to,  19,  28 

bead  for  electroscope,  28 

condenser,  33 
Suspension 

of  electrometer  needle,  9,  u,  12 

quartz,  n 

phosphor-bronze,  12 


244 


INDEX. 


Table 

of  radio-elements,  222 
Temperature 

effect  on  emanating  power,  199 

of  volatilization  of  emanations, 

201,   202 
Tests 

difference  between  electrical  and 

photographic  156,  184 
Theory 

of  ionization,  74 

of  successive  changes,   187,  218 

disintegration,  218 

of  structure  of  atom,  219 

to  explain  rayless  changes,  220 
Thickness  of  layer 

effect  on  radiations,  161  et  seq. 
Thomson,  J.  J. 

charge   carried  by   cathode  ray, 
43 

theory     of     electric     charge    in 
motion,  53 

relation    of    negative    ion    and 
electron,  95 

charge  carried  by  ion,  125 

theory  of  structure  of  atom,  219 
Thomson  and  Rutherford 

theory  of  ionization,  74 
Thorium 

discovery    of    radio-activity    of, 

133 
photographic     action     of     rays 

from,   133,   156,    157 
ionizing  power,  133 
complexity  of  rays  from,  143  et 

seq. 

absorption  of  7  rays  from,  164 
separation  of  Th.  X  from,  183, 

185 

loss  of  activity  of,  186 
recovery  of  activity  of,  186 
theory  of  successive  changes  in, 

188 
emanation  from,  190  et  seq. 


Thorium 

continuous   emission   of   emana- 
tion from,  196 
period    of    rise    of    activity    of, 

196 

emanating  power  of,  198 
de-emanation  of,  199 
condensation  of  emanation  from, 

201 
excited  radio-activity  from,  205 

et  seq. 
transformation  products  of,  210, 

211,  217 

Thorium  A,  210,  211 
Thorium  B,  210,  211 
Thorium  C,  211 
Thorium  D,  211 
Thorium  X 

discovery  of,  183 
chemical   separation   of,    185 
decay  of  activity  of,   186 
as  source  of  emanation,  203 
Toepler  pump,  35 
Townsend 

recombination  of  ions,  91 
relation  of  negative  ion  to  elec- 
tron, 95 
Transformation 

in    radio-active    bodies,    187    et 

seq. 

products,   189 

Transformation  period   (see  period) 
Transformation  products 

of  thorium,  210,  211,  217 

of   rapid   change   from   radium, 

214 
of    slow    change    from    radium, 

215 

of  actinium,    216,   217 
of  uranium,  218 

Ultra-violet   light 

discharge   of  charged  plate   by, 
107 


INDEX. 


245 


Ultra-violet   light 

production  of,  107 
ionization  by,    109 
fatigue  produced  by,   1 1 1 
Uraninite  (see  pitch-blende) 
Uranium 

discovery    of    radio-activity    of, 

127 
photographic     action     of     rays 

from,  127,  128,  156 
discharging  power  of  rays,  129 
ionization  produced  by,  130 
constancy    of    radiations    from, 

132 
complexity    of    rays    from,    141 

et  seq. 

a,  /3  and  y  rays  from,  142,  143 
relative      ionization      by      rays 

from,  153  et  seq. 
homogeneity    of    /3    rays    from, 

159 
absorption    of    rays    from,    141, 

iS9»  J63,  164  et  seq. 
separation  of  Ur.   X   from,   183 
loss   of  activity   of,    183,    184 
recovery  of  activity  of,  183,  184 
theory  of  successive  changes  in, 

187 
devoid   of   emanation   or   active 

deposit,  208 

transformation   products   of,  218 
relation  to  radium,  218,  222 
Uranium  X 

discovery  of,   183 
chemical  separation  of,  183 
decay   of   activity   of,    183,    184 
photographic  action  of,  183,  184 
radiations    emitted   by,    184 


Vacuum 

production   of,   35 
Varley 

source  of  ultra-violet  light,   109 
Velocity 

of  cathode   rays,   48 

of  Lenard  rays,  50 

of  canal   rays,   51 

of  Rontgen  rays,  53 

of  ions,  98  et  seq. 

of  ions  in  flames,  115 

of  /3  rays,  159,  176 

of  a  rays,  159,  177 

Walker 

theory    of    quadrant    electrom- 
eter,   13 
Wax 

use  of  different  kinds  of,  as  in- 
sulators,  19 
use     of,     in     making     air-tight 

joints,    38,    39 
Wehnelt  interrupter 

use  of  with  X  ray  bulb,  68 
Willemite 

phosphorescence    of,    under    ac- 
tion  of  rays,    158 
action   of   emanations   on,   203 
Wilson,   C.  T.   R. 

ions  as  nuclei,  118,  121,  123 

Zeleny 

mobility  of  ions,  99 
Zinc  sulphide 

phosphorescence    of,    under    ac- 
tion of  rays,  158 
scintillations    produced    in,    by 

a  rays,  158 
screen  made  of,  158 
action  of  emanations  on,  202 


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